At some point will all the movies ever made be on demand?

Question

And you pay flat monthly fee to see them anytime you want...

Answer 1

No, for one simple reason: The last copies of many old classics from the early days of cinema are rotting on the reels.  Curators can't attract enough money and people to save and restore them.  With limited resources, it's a battle against time, and time often wins.

As for your pricing scheme, it would work for all but the hottest titles.  However, the price would have to be so high that they'd make big money on the many casual users (like a gym membership), without losing much on the fewer heavy users.