The recent introduction and success of IEO proves this right, because the exchanges validate the projects, so investor feel safe to invest their money.
Also a research done by exchanges is still no guarantee that the listed projects are legit and even if they are legit there's still an option that they fail. The exchange can't do anything if the developers are bad. In Germany, we had a big ICO, they collected 40 million $ advertised on big German websites and finally the project is now almost failed. It's likely that every exchange would have listed them because it seemed like a good project but it wasn't.
Why are you sure that exchanges will be a guarantee for successful projects?