To be honest I already had this concern when UE4 was announced with just 19$ to download it and get an account here.
I was really worried that there wouldn’t be enough support available for specific professional questions like in the Unity forums.
These can only be answered by a selected few experienced community members, so if there are tons of beginner questions it would be hard to filter out the ones interesting for them.
In UE3 times this wasn’t such a big deal since many people would just give up because of the high barrier of entry of learning how to do basic things in UE3.
However I think that Epic managed to solve this problem really well so far by having their Staff answer the professional questions here and especially on AnswerHub.
So my hope is that they will be able to continue to do this, even with the influx of new users. Then I think making UE4 free is a good decision to get a larger share of the market.