I think these hypothetical apps should be based on html embeds. The big tech companies are not going to remove the ability to do web embeds of their posts/video

I think these hypothetical apps should be based on html embeds. The big tech companies are not going to remove the ability to do web embeds of their posts/videos/tweets, because embeds are good for bringing traffic in. With those services that don't have RSS (Youtube does), you will still need a little scraping just to discover the URL of new tweets/posts in a given channel. But it would be just basic scraping and probably more manageable compared to something like youtube-dl.