I am not saying that we are any better; there are a lot of things we don’t know that will be taken for granted in 200 years. If there is still a human race.
But that's not particularly stupid. Bad smells are certainly a proxy for diseases to some extent and avoiding them/removing their sources would also decrease the likelihood of getting sick.
No, they are not. Because smell and germs are completely unrelated. There is a limited case where they are correlated (don’t play with organic waste or too old corpses), but even then it is not really helpful. This is why it took so long to understand how to stop cholera: you can have things in your water even if it looks pristine. The air is just a red herring.
It is not even helpful with airborne pathogens, as once they start spreading among humans all bets are off.