You can today no? You can buy or rent a single movie / tv series from apple tv, amazon etc. problem is most people don't want to buy each thing they want to watch.

Sometimes you can. But there are also shows where the only (legal) way to watch it is on a particular streaming platform where it is "exclusive".

You mean the "license while they feel like it" kind of purchase?

If I could pay for individual TV shows and actually own them I'd definitely prefer that over the disaster we have today. Buying a blue-ray and ripping it is not very practical and it's by design.

Netflix (notoriously) does not license most of their content this way. You can't rent/buy Stranger Things on Apple TV, no matter how much you're willing to pay. If Netflix acquires Warner Bros, I expect this restriction to extend to that content too over time.