And I'm sure we're gonna be running into almost the same problems that Skyrim ran into when they tried to do paid community content.
That was down to the way Valve, not Bethesda ran it. What we will probably see are those more high profile modders releasing stand along UE4 games, not mods.
As of this moment, UE4 is making hardly any money.
UE4 may not be, but UE3 has made a shit ton of money for Epic, along with a couple of high profile releases. If they weren't making money UE4 wouldn't be free.