I’ve recently tried to use peertube and I think it could improve a lot if it showed all the content in all instances. But instead you have to look around many instances to try and find something you like. Then there’s other thing, it can’t suggest content if it doesn’t know what you like and without sharing the data between the instances it doesn’t know anything about anyone. If the user data was encrypted and shared between all sites, when you log in it could use the now decrypted user data to suggest content. Or maybe it can share the data with third parties, I don’t really know.
Most peertube instances are pretty bad right now, so if one instance decides to follow everything else, that instance would be full of trash.
Also, as with a regular webpage, it doesnt make sense to link your instance to every other instance. Sure, by doing so, you increase the likelihood that there is some relevant content on your site, but it also becomes a cespool.
Rather, it makes more sense to be mindful about the instances you follow. Follow only those who are related to the content for your site. Your site should be thematic and have a vision. Follow the sites who shates a similar vision.