If they do end up playable, I do worry that everyone will just use Octolings and the Inklings will be forgotten.
Well we have to deal with that then.
Restricting the use of Octolings in any way, would be a jerk-move in my eyes.
Especially if it is done, just to force people to keep using Inklings, in order to keep their popularity up.
But that's not how it works.
If people prefer Octolings, then let them play as them freely.
And if you prefer Inklings, then you have the freedom to stick to them.
What exactly is the problem with online being swarmed with playable Octolings anyways?
They have the exact same moves. They are in no way superior or inferior to Inklings.
So yeah, what exactly is your problem?
Or is it really just your fear of Inklings being less popular, because you prefer Inklings?
Well again, you have to accept it.
You can't force people to play in a way, they don't want to play.
Each and every person has the freedom to make their own decisions, on what they themselves enjoy.
And if they choose Octoling, if that option was a thing, then so be it.