Goog is 100% not going to delete anything that is driving any advertising at all. The videos are also useful for training AI regardless, so I expect the set of stuff that's deleted will be a VERY small subset. The difference with email is that email can be deduplicated, since it's a broadcast medium, while video is already canonical.

I expect rather than deleting stuff, they'll just crank up the compression on storage of videos that are deemed "low value."