I see lots of people who really only know* one system saying that they only need one system.
*I'm using know as in: have played at least short campaigns in other systems. Just "I read it" or "I tried a one-shot with others who hadn't played it" isn't enough.
But none of the people who know multiple systems are saying that. At the least, they are saying they want a good generic system like Savage Worlds that is designed to handle a lot of different games, and the others who know multiple systems are saying multiple systems are goods.
I have to say that since it really seems to be in a minority that people who actually know multiple systems only want one, that perhaps the people who only know one type of system and say that's enough don't have the information to make an informed choice.