It’s definitely sustainable, which is why every movie studio now is doing their own streaming service instead of putting their content on someone else’s.
You kind of answered your own question, because every movie studio will want a own streaming service and as soon as the total cost is too high people start to pirate in masses again.
There isn't enough people who can afford to pay 50$+ a month to watch all of that but people want to watch it so I think you are very wrong there, as soon as the user experience gets awful or expensive enough piracy numbers skyrocket.
How do you figure? I believe Netflix is the only service to actually make a profit off streaming and they have something like 250 million users. The rest of these companies are just dumping money into a bottomless pit and trying to outlast their competitors in the hopes of gaining enough marketshare.