The bigger issue would be search and discovery, especially since I can't think of any obvious way to prevent services from spoofing engagement data to influence rankings.
Absolutely, as long as there was widespread agreement in both the distribution mechanism and important video features like closed captions.
> The relatively closed nature of YouTube isn't necessarily something you'd want to replicate.
No, but the consistency and ability to get everything in one place is pretty vital, and is something you pretty much get for free from single centralized sources. The closed nature is obviously one of the huge downsides.
> The bigger issue would be search and discovery, especially since I can't think of any obvious way to prevent services from spoofing engagement data to influence rankings.
Yep. Another big one is the cost of serving videos.
But the more important point is that even if all the many dozens of mostly small channels I subscribe to were on some other paid video service, I wouldn't want to track down and check in on all those services. Even just 3 would be a vastly worse experience than YouTube, especially since there would almost certainly be huge variation in the features and quality of their various web, mobile, and TV apps.