Why is everyone so sure this is evidence of cognitive problems? Isn’t it just as possible that as he is aging, he’s losing a bit of the control he has had over that speech impediment, and that his cognition is not significantly impaired?

Mostly because that's not what everyone saw at the debate. That wasn't a speech impediment resurfacing. He was clearly having trouble with his thoughts. Plus, had it been consistent with nothing more than Biden's previously diagnosed and long-present speech impediment coming back, then that would have been pointed out - ad nauseum - by the White House over and over again, with experts lining up to say that.

Plus, I'm not sure it would matter if that's the reason Biden has lost the ability to consistently speak effectively without a teleprompter. Oh, sure - much better than being partially senile, of course. But it's still really bad. Running for President is a different job than being President. You could (probably) continue to do the job of being the President even if you were struck mute after being elected. But being a Presidential candidate requires being able to communicate with voters using the spoken word - it's sort of a BFOQ of that job. You have to be able to talk to people - general public speaking, debates, town halls, conversations with donors, interviews with reporters, meetings with surrogates, etc. If Biden can't do that going forward, then it's really hard to see a path forward for him to beat Trump.