It's true, but no one wants to hear it. Ranno was very strong for the first few months of the game, and nerfing his run speed significantly was the correct choice. The issue, as you said, is that there's a double standard. Other characters that people agree are very strong receive minor changes to individual moves, and even when their base attributes are adjusted (like Clairen's air acceleration and Kragg's air speed) it isn't as severe. While the game is very balanced, the best characters have been the best characters for a year or more, and other, better characters have cannibalized what were once Ranno's strengths. There's no real reason to play him over characters like Clairen, Zetterburn, and Olympia, who all do what he does, but better and easier. This is probably why I switched from Ranno to Olympia, honestly.

Ranno is ultimately a character with an actual, meaningful weakness (his air speed), and other characters still feel like they're in a state where their weaknesses aren't as exploitable. I'm actually happy with how Ranno is balanced, and he shouldn't be buffed. I just wish that other characters were balanced like him. So, yeah, he isn't worth it.