There was some discussion about this in another post some time ago: https://lemmy.world/post/238970
To me it seems there's not really a consensus either way on Lego competitors at the moment. I for one am of the opinion that splitting off into multiple communities would not be a great idea for now, seeing how little content there is so far overall anyway. Having a community simply called "Lego" makes for the best discoverability right now, and allowing compatible brands as well just increases the chances of growing the community healthily. We can always change the rules if it becomes a problem.
Edit to add an answer to the other question: I do have quite some experience with alternative brands by now, I've written a little about that here. Not so much with technic-specific alternatives, but from what I'm hearing, that generally positive experience seems to extrapolate to technic offerings as well.