Yes, that is the question to which neither of us has an answer. Streaming deals happen as bundles, and my understanding is the opposite of yours: the old stuff gets thrown in as a piece of the bundle to help make the catalog bigger, while new content actually sells subscriptions. In other words, the back catalog has some marginal value to the streaming service (and the studio), but it's the new releases that actually sell subscriptions.
Releases of "The Mandalorian" got people subscribing to Disney+, and new seasons of "Game of Thrones" had the same effect for HBO, for example.
In other words, a bundle of 100 movies for $10 million could be attributed as "$100k per movie," but that's almost certainly wrong. More likely, that bundle is a combination of something like $5 million for one movie, $1 million for a few others, $100k for a bigger set, and all the way down to $1000 or less for the remainder of the catalog.
I dont think any streaming service would be viable with a catalog of only original productions <1 year old.