The ideas category is for exactly what you used it for, no question, and you shouldn’t have to feel bad about posting there. Yours was actually one of the better ideas posted too to my mind.
One user thinking the ideas category should be something else entirely and hijacking the thread about your idea to say so doesn’t change things. They should have come here, to site feedback, and started a discussion, as you have.
Personally, I’d keep the ideas category as one rather than split it: you don’t need to be a game dev to have an idea, or invite others to critique or expand on it. And being a dev doesn’t mean your idea is going to be better or worse than anyone elses (it might just be a little more practical to pull off, or might not). I’m with Lt. Doolitle on this: “the concept is valid no matter where it originates.”
From a dev point of view it’s true you could look at ideas negatively as a begging bowl, but why not think of it as free market research instead? Look through the posts, see which are faved most - those are potential “customers” for your work.
There’s already a Game Design Discussion category for devs to get into the weeds with an idea. And if it’s something they are working on, then it reasonably belongs in Projects.
If it were possible I think an option would be to improve things would be to require ideas posts to be well defined (like yours was) and more than just “{commercial-game} but fat”. Perhaps a first step would be to change the ideas category pinned message to explain this, and also that the chance of anyone picking it up are slim?
Oh, and that “disgruntled game dev” - they’ve never produced a game here. So there’s that; at least you’ve contributed ideas.