Earlier quoted context omitted.
Are you doubting that homeless people tend to have worse hygiene? Or that people don't want to use public transit when it smells bad?
What I doubt is the notion that the number of people who don't use transit because of the homeless is statistically significant. The people who seem to complain about it most are also the sort of people who won't ride transit anyway and just use it as an excuse to stay in their cars.
Directionally, I think it's reasonable to assume that bad smells negatively impact ridership. It seems all we disagree about is the magnitude. Given how few people ride transit now, even a small number of folks who avoid transit due to smell seems likely to be a decent sized percentage of overall ridership.
I'd bet that ridership would go up 10+% if you could magically remove any hygiene and perceived threat issues. Although now we're well into the territory where we are each just projecting our own opinions, because AFAICT neither of us have data to back our assertions.