Sure, I could see them tightening up the "sharing books" stuff. Maybe. Then again, if you need a particular level of sub to use the VTT, maybe they could simply have a tier of sub where you are paying to access the VTT.
I look at Fantasy Grounds, for example. I have an ultimate license, so, I can share whatever I own while the game table is running. And the players do not need to pay to play in my game, since I can host free version players. But, if you want your own books, you need to buy the standard license (I think - I might be wrong here).
So, I could see something like that going on. It depends on how integrated D&D Beyond is going to be with the Sigil. If it's sort of one stop shop - the DM has a master level sub and everyone can access those books, with an additional cost for having whatever sub for hosting Sigil games - it might not be too terribly different than the way Roll 20 or Fantasy Grounds already works.
Then again, maybe some players could actually pry open their wallets once in a while and pony up and help the DM out. That might be a nice thing to happen once in a while.