The theory with the theatre rule, as I understand, was that the studios were using monopoly position to limit what movies a theatre showed - i.e. only theirs. In a small town, with room for, say, one or two theatres, the audience would never see some movies because the theatre only played certain studio’s films. Studios would make it impossible for independent film makers to distribute their movies to enough theatres - essentially a monopoly action, using their size to force the little guys out.
IIRC, even with independent cinemas, the studio tried to twist arms to say for example 'If you want this blockbuster movie, you have to agree to also show these other movies".
With the internet - who cares? you can get any streaming service you want anywhere in the country (if you have bandwidth).
Yes, there’s an element of the theatre dilemma - to see all the good movies, you would have to sign up for a dozen streaming services, maybe. But at this point, I think the government is just going to let the market sort it out. I doubt many theatre goers used to spend $120 a year on, say, Sony pics. So Sony would have to work really hard to produce enough hits to justify $10/month streaming. Even worse for networks and their series - we used to get those free.
I think the ideal would be a setup like a Blockbusters, where almost everything is available, and your subscription is distributed to the studio/artists based on your choices. (I believe the music streaming services work this way).
TO be more succinct about the OP question - I have trouble seeing how the government could use the monopoly laws to force this sort of setup; Disney may have a lock on children’s entertainment with its back catalog, but most others are at the whim of how good their newest product is. Monopoly is about restraint of trade, forcing the others out of the market through domination; I don’t see multiple streaming services being defined as that. What you may see is streaming services being told they cannot have “exclusives” in their contracts with studios - i.e. if Make-a-film productions makes a film, XYZ streaming cannot say “we will only take it if you don’t let any other service offer it”. That to me seems to be a form of domination - Netflix or Amazon making it impossible for Bob’s Streaming to get an independent blockbuster as well.
The theory would be that then the film maker would license to any/every service. That’s just good business. (Assuming the rules also included no paying extra for exclusivity) Disney could make a fortune, for example, licensing their kids movies to Amazon, Netflix, and everyone else. However, if the streaming service belonged to the studio, why would that matter?
To make this happen, we would have to see an example where small producers were being crowded out. But with the ability to start their own streaming service or pay per view, it’s hard to imagine this happening.