Well one thing is you can't really compare RPGs (which I'm assuming based off of the mention to FF15) to a shooters, as they're completely different ball games, especially when one is more focused on being a single player offline experience and the other is about playing online multiplayer and co-operating with random players to win matches. That's like comparing a house with a hotel really, one is where you live and pay a lot of expenses with bills and you actively manage to keep in a clean and tidy state yourself, and the other you pay to stay there for a certain timespan and don't have to worry about utility bills, food (where it is included) and cleaning. It gives you two vastly contrasting objects and is trying to compare them like they work the same, even though that is wrong and will never give a justified argument for changing one to be more like the other.
As for the argument in general, Splatoon has worked with the current ranking system, with the only issue being as time went by people naturally ranked up who shouldn't have due to less players, lucky wins, getting carried etc. due to how the game did ranking with points being distributed. B through to A+ does +10/-10 for wins and losses, with +12/-8 for 0-30 and +8/-12 for 71-99 (100 being rank up). This initially worked at launch, where the max rank was A+99 and far more people played, as people got stuck in ranks that reflected their natural skill level (I was stuck in B for a while before getting to B+ and then A- at the 2.0.0 patch in August 2015 that added S and S+). As I stated, when people stopped playing (or played less frequently), it created skill gaps, so people who were firm Bs climbed a rank or 2 and the like (A- to A+, A to S, B- to B+ etc.), and this naturally got worse as time went on and more people dropped out. As this happened, the average skill level in each rank dropped, and people who were managing with a 55% win rate (where they climb up slowly, but then have a bad losing streak and stay in that rank) then start winning more against those who were pushed up with the skill vacuum, which in turn pushes more and more people up against others who clearly out-skill them, which then makes it worse and worse.
On top of that rank and level were stored on the Wii U save file, so we had save scummers who would load an old file they backed up if their rank dropped to effectively wipe out their losses, and then they got into ranks they didn't deserve which again pushes up others. Then we had Squad Battles where people who reach S/S+ would not have their ranks change, but anyone below that would, so then we had true S/S+ players helping their friends reach S/S+ by carrying them to A+99 (max it goes to in Squads), then getting a fluke win in their ranked match and hitting S. Now granted those players weren't staying in ranked, but just did squads afterwards and didn't harm it, although some did carry on into ranked and caused havoc with the S skill level. There are severe flaws in Splatoon's ranking system, but most can be fixed with the knowledge the devs have received from the way people played 1.
So yeah, the ranking system needs adjusting, but adding unnecessary flags and checks for whether you're actively contributing seem unnecessary, especially if (as mentioned earlier) people try exploiting them by inking turf and not going anywhere to fight to stay alive as a "contribution". Reminder that staying alive isn't a contribution if you lose map control and get pushed back and knocked out. Someone in Splatoon 1 could go 21-1, and lose their team the match because they weren't paying attention when the Rainmaker was pushing right past them and they ignored them for kills (which happens quite a lot in Rainmaker) or weren't paying attention. Then they basically get off scot-free from the loss of points because they game thinks they were the best contributor with kills, inking and not dying when their teammates could have been setting up pushes and getting 2 kills in a trade and earning points that would have won the game if the 21-1 player had stopped the push. See? It's situations like that where people get screwed unfairly with the points system if it worked with key flags. Meanwhile you could have a 2-10 player who pushed the objective a lot (which is a key flag, but 1 of 4 or 5) and caused distractions with their subs and specials which sets up an opportunity for their team to get the winning push and they get next to no points.
There are far too many roles to play in a shooter, you can be support, aggressive, defensive, all around or some other player who harasses the team and prevents them pushing forward. Combined with all different weapons and ways to work you can't fairly add markers without bad repercussions occurring.
The best way forward in Splatoon 2 is to keep the C- to S+ (no S-) ranks but make it tougher to rank up. S does +5/-5 then +4/-5 and finally +4/-6 at the top of the rank. If these sorts of points start at B+/A- with B and lower being the only +10/-10 standard ranks (C- and C being a little easier), then ranking up will require more wins, and thus more "skill". At the end of the day, skill is what determines your rank, but Splatoon 1 was a little too easy to rank up for most of the ranks. If you aren't ready for a rank, you know because you'll get crushed by the other teams nearly every match and drop down a rank immediately, and eventually you'll learn the skills to hold your own and earn the next rank. As for other improvements, Squads should have a separate rank where you can form a team that you invite people to and you can use that to squad together and the game has a "team rank" that starts at the average rank of the players in the team, and goes up and down based on wins/losses, but has no overall impact on individual player ranks. That stops the carrying aspect (save scumming should be gone based on the Switch not allowing save copying) and makes it so you have to earn your rank once more. S- doesn't need to exist, and if you couldn't lose points when you hit S and S+ ranked would become redundant as everyone will reach S+99, as well as people starting to troll and not play ranked properly and rage quitting etc.
Ridiculously long message, but had to consider many things and put forward a few mock up examples of why flags on certain performance points is a horrible idea for this game.