Competitive Splatoon has been set off, it has been getting somewhat of a strong start with discovered data on weapons, clothing, etc. Almost everything is set up well. All there is left to do is to wait till August to get private rooms and to decide what should be the primary game mode for tourneys.
The current suggested one is Splat Zones, as it is considered more competitive and the game says it's more competitive than Turf War. I will prove later in the OP that what the game says is wrong. Turf War is considered casual and is "non-competitive". I will prove why this is wrong. Tower Control is considered boring. This is an opinion, a lot say it's fun and even more exciting than the other modes. I will prove why both of those opinions are fine.
Now here are my reasons why Competitive Splatoon should use all Game Modes:........
I've read over all the pages of responses here, so I am not going to argue with the opinions of others. I do agree with your initial post though. Here are my thoughts after playing a lot of turf wars, splat zones, tower control, etc.
Turf wars, can it be competitive? Of course it can. But only if it is setup that way. The current Turf wars settings are very casual compared to any of the ranked modes. You can goof around in Turf wars and your team can still win. You can goof around in Turf wars, team lose, and still get points. You can be awesome in Turf wars, your team lose and you still get more points than the top person of the winning team (done that myself a lot of times).
So I think the current Turf wars settings is good for new players and those that don't have a lot of free time to invest into playing. That would target busy adults or busy school kids, etc. Very young kids that don't want to worry about too much team oriented strategy, just want to have fun.
Now, you want a competitive Turf wars? I saw this during splatfest. Teams were a lot more aggressive when I played. I could always tell apart the team with strategy from the team with the goofiest playing. :) Still, even during splat fest, there was not as much pressure to win because even the losers got half the points of the winning team for just trying.
If Turf wars was put into a ranked mode where the losers got nothing, then it would become a competitive mode. There is strategy to be had in Turf wars, it is simply push the other team back to the spawn point. If the other team can't leave the base, then no amount of last second painting will be enough. I know there are plenty of video examples of the last few seconds of turf wars turning over an entire map for the win, but that is basically the same strategy of pushing the other team back. Give Turf Wars a ranked mode and players will find strategies that work.
With my opinion out of the way on Turf wars, I agree that all modes should be opened for ranked mode. There is a different strategy to be had for each mode of Splatoon and locking into one every week, I am still on the fence about it. For one it gives you a week to practice the mode and what it takes to win. On the other, if you don't like splat zones and prefer tower control, you end up just not playing or given a partial effort when playing online (because it is not your favorite mode).
I think the Splatoon player base is big enough to have a "Splat Zone", "Tower Control" "Turf War" separate ranked modes for the players. Why not be an A+ in Splat Zones but only a C+ in Tower Control?