It isn't sustainable in the context of their desire to pivot to being a streaming service, along with their desire to blow resources on repeatedly redesigning the apps to make the UI slower and buggier.
The change to $750 just took effect July 1st. I suspect we're going to see some big discounts down to $400 or less around Black Friday and periodically at other times of the year.
pCloud, a cloud storage provider who offers lifetime plans, uses a similar strategy.
Basically I think they are putting the $750 price out there a) for anyone who is dumb enough to pay it, and b) as a way to make limited-time discounts and the annual plan seem like better deals by comparison.
$750 is insane considering how little they care about user feedback.