I had assumed that the reason old movies (and “old” now apparently means pre-2010) keep getting dropped is that the streaming services don’t make as much money on that “commodity.” Or, alternately, there must be some business reason to make sure your subscribes are only watching all the “new content.” Perhaps it makes a streaming services more attractive to the studios because they can guarantee a larger audience? This is not my area, so these are all assumptions.
This is why I subscribe to Criterion about every other year.