That might be true to some extent but the flip side is that really good members, knowledgeable and helpful members that have been helping others for free might not join either because they could argue why should I now have to pay to help people when I have been doing it for free.
I think the risk of losing really good people after introducing fees is higher than the risk of having more trolls if there are no fees.
Dealing with and booting out trolls when there are no fees is much easier than trying to replace really good, knowledgeable and helpful members after introducing fees.