Yea, that's pretty much what it came down to. Look how many people noted how Shadow effectively is both Knuckles and Metal Sonic wrapped into a single character, or how Blaze is eerily similar to Knuckles in terms of personality and role. So because of that, Knuckles' own personality got severely diluted and reduced to typical muscled dumb guy comic relief. I actually asked this question on reddit, and people agreed the cast aren't exactly three dimensional, and that they don't have to be. I think people just want them to be entertaining to watch, as opposed to just being caricatures of themselves. Of course, what people consider "entertaining" varies greatly as well.