I don't want to pay for all of them separately. Most people don't, they just choose one or two, and call it a day. I have zero clue about roku as that's not available where I live, which is another can of georestricting worms you don't want to get me started on.
Having one provider that offers all streaming services through a single unified interface for one flat fee would be a win win. Both for the user and the providers. Who are right now cannibalizing each other's user base.
The good thing about streaming over cable is that with streaming they can tell exactly what you watched and for how many minutes. So the cut of amazon / netflix / etc can be decided on the user level. If one user watched 240 minutes prime content, and 60 minutes netflix content then 20% goes to netflix and 80% goes to amazon. As opposed to currently where the user will simply only sign up for prime and netflix gets zero. This is the only path forward for multiple streaming services to be sustainable long term.
The other outcome is the bigdogs eating the other smaller ones, while they themselves also suffer from continuous attrition as customers flip flop between services, or pause their subscription while their favorite show is not getting new episodes.
This has nothing to do with ads, it has to do with access, ease of use and practicality.
I don't see any of those issues you mention, its really no big deal to pay for whatever you want to in whichever way. If I wanted to have as near as 1 account as I could, you can help that to a reasonable extent, i mentioned roku, but i think you could with apple, maybe more , i don't really care about it so I haven't research it much at all. Theres also services that handle your subscriptions at least for cancellation made easy, not that is hard with each service anyway.
I find it unlikely that such a percentage service will happen, though it might has yet another one but I don't think it will as a substitute. That would be an absolute catastrophe, and would mean mayor collusion etc etc, high prices to the moon in little time.
Hulu in many ways was just that idea, with numerous providers in it at the start with a mind of expanding. What an effin mess hulu turned into. I used it for a good while, it was decent for one hot second, then infighting started, and the service declined, I don't know if it recovered. Don't care to find out.
Theres also apps that cobble all your contents into one, but again the backbone is whatever services you are paying separate.
I wouldn't expect a spotify of video, content producers sink 100s of millions, adding to billions into said contents, and if that ever came to be, one app monopoly style if would probably be 100$ plus a month, or some such. Music yes can go high in cost, but can also be next to zero, and be good music anyway. Anyway i wouldn't know specific of your country. But really paying for a few services, having some on and off for some specific programs is not hard at all, and works very well. There is no need to recreate cable in the name of such trivial issues.