People just have this bizarre idea that there is some "limit" to how influential a particular move can be before just being considered "broken" automatically, with zero regard to whether the game is actually more or less deep or enjoyable without it. If the ability to switch Pokemon were attached to a move, and every single team absolutely 100% had to have at least one Pokemon with that move, these people would say that that move is broken even though the strategic options created by the ability to switch are obviously immense, and completely integral to the way we play Pokemon in actuality.
The fact that many of us view Stealth Rock as this sort of "alien" thing that mysteriously invaded in 4th gen (but was completely absent before then) doesn't say anything about its brokenness, no matter how much impact it has on the game. Neither does the fact that it's a "move" that doesn't have a very "move-like" level of influence on the game. Now okay, if you think that there is innate merit in removing all elements in Pokemon that we happen to view as "alien plus influential," go ahead and make that argument. Likewise, if you think moves should just inherently all have a similar amount of strength for some weird aesthetic reason or something, go ahead and say that. Don't come in here and try to argue that either of those things say anything about the move's brokenness, though. They just don't, and it's an absurd argument.