Well...it all depends on context.
There are two Pokemon (to clarify, 6v6 Single Battles) players I know that will remain unnamed for the sake of this discussion. Both of them are respected as strong players within the community I'm a part of and they've done a lot of great things in our tournaments. One of them exclusively plays a team archetype called Stall and one of them exclusively plays Offense. There are other archetypes like Balance that they can technically play but will always see not-as-good results with.
Stall is an archetype that dedicates all six party slots of a player's team to exclusively having as much in the way of defense as possible. You have blanket answers for physically and specially offensive Pokemon, plus boosting moves, more specific Pokemon that patch things up, and so on. There are such things as Pokemon that do well against Stall teams, but they're usually not so simple to use in the matchup because the sheer amount of softer or heavier answers Stall usually has to them so the players who use them need to whittle down certain key Pokemon in a Stall team before this anti-Stall Pokemon can do its job effectively.
Offense is a much less strict archetype. There are a lot of different subsets of offense, between Dual Screens, weather, and so on, but the main goal is keeping unrelenting pressure on the opponent at all times to prevent them from ever getting the time to do their thing. There are still a lot of things you absolutely need for an offense team to thrive. Some fast Pokemon, one or two utility-focused ones, and some Pokemon specifically meant to beat those that have a good matchup into offense.
The reason I bring both of these up is to say that there are Pokemon that specifically do well into each of these archetypes. While you usually can't bring too many of them on a team without it suffering in other areas, that doesn't matter when you know who your opponent is. Trick Room might be an awful matchup fish of a playstyle but if you know your opponent is bringing a teamstyle that focuses heavily on Speed then it's going to give you a huge advantage. Running more than one pure Stallbreaker might not be ideal for most teams but if you know your opponent is likely running Stall then it's not a huge deal.
These two people can try running other team archetypes, but their familiarity with Stall and Offense and the tools those kinds of teams tend to have make it not as straightforward for them to follow how to build teams with other archetypes. This isn't to mention how much harder it is for them to respond to high-pressure situations with them in practice. However, there still is the benefit that if someone doesn't know who you are then you have a much more refined set of knowledge on one specific part of the game that you can now use to your advantage.
It is worth noting that this lack of flexibility can cause problems in another big way and one of these two players got the short end of the stick. The latest generation of Pokemon games introduced a bit of power creep and a bunch of broader mechanical changes which have all lead to a drastic shift in the state of the game.
In Generation 9, you will only ever see teams that are some variation of offense 99% of the time. Stall teams still exist but are in a much, much worse spot as your building options are even more limited than usual and opponents have many more ways of prying at your defenses. I've seen some very high-level Stall players play against some no-name mid-level players and have incredibly close games that would not have been possible in previous generations. I don't know where either of these players are at or how they've been effected by this but you can already guess which one got of better than the other. Bottom line is that it could have been either of them.
This much leads to a very obvious parallel in Splatoon where if Splatoon 4 comes out and in your case the Splattershot Jr. or Shooters in general got some big nerfs, then you're going to end up working a bit harder than others might to hit the same level as you hit in Splatoon 3.
The thing with Splatoon is that there are a lot of weapons that don't work on quite as strict of "counters" as one another. If you know your opponents are bringing a Brella you can bring a Wellstring to make their life miserable, but how many examples of that in the game are there? Not that many, let me tell you that. Many of them are weaker, but still playable if you're cautious, like how with maybe something like a Dynamo Roller into a Brush you can still keep your ink tank high and try to call out the timing they use to approach you. This is assuming that a weapon you play doesn't have a kit that shores up its problems like Foil Squeezer having tools for sharking and longer-ranged threats which are its two biggest problems otherwise.
This is also assuming your teammates aren't there to help you with those problem matchups which they often are. This is a team game for a reason. In many of these cases you don't have to play to the same spots on the map as these other weapons are patrolling or if you have to, you can be sure to wait until someone else can go with you.
Shooters are in a pretty unique case where the basis of the entire class centers around making the player feel like they have options. They have some of the simplest and quickest movement even if not the absolute best, very good painting, always will have a good kit, can play to practically whatever range you want, are never going to get nerfed out of the meta and so on. Reflexes and skills you gather from one outside of some positioning or aiming if your range value changes too much will generally transfer from one Shooter to another. If you truly want to stay with only Jr. then this matters a bit less but is worth saying.
Let's go back to the example at the start of this post. Someone is going up against a pickup you're playing with and the only person they recognize from your team is you. We don't need to go over what happens in solo queue for aforementioned reasons. In this case with the pickup, chances are it's happening either in a meta where Bubble is a top tier special and is probably reasonably viably even when counterpicked or in a meta where Bubble is not a top tier special and there are more pressing things for the opponents to try to counterpick. Even in the rougher cases of those...you're still a Splattershot Jr. You can still give very good paint and bomb support at the absolute minimum.
So, in the context of maining a lot of Splatoon 3 weapons, I don't think flexibility is an absolute must. It can help you fit onto more kinds of team comps or adjust to shifting metas a bit easier but both of those are only at their most pressing as you get to an especially high level of play that I don't think anyone especially active on this website has reached yet. It probably will help in the case of some main weapons but I don't see this one being a case where that's necessary.
Also...
as if I'm not as worthy enough of a player compared to my peers to be consistently good at something more unique and exciting to play against.
It took me a day for the vanilla Splattershot to become my strongest weapon and it instantly boosted my skill level by like 3 divs, .52 Gal is .52 Gal, and I've gone on solo queue and won games with the Bloblobber even though every kill felt like an accident that my opponents walked into, not to mention things like Tri-Slosher or Octobrush. If people are saying or implying all this about a
Splattershot Jr. player with the amount of other overtuned weapons in this game then they're wasting their energy I don't know what to tell you LOL
no matter how much time I put into dedicating mastering the full kit, there will always be someone who just says it doesn't matter because I can throw bombs to solve any problem.
People who think this way do not understand the matchup at all. If it were truly that simple then you'd see wayyyy more Jr. players because every game would turn into both an extremely basic and effective flowchart that wouldn't have a lot of counterplay. Even a rolled Splat Bomb directly on top of you is very reactable and I'd still have counterplay to talk about even if it wasn't
