I have two accounts from my youth with mostly fansubs and "funny" vids. (A "Everybody Draw Mohammed Day"-Video that, 11 years after its upload, got band in Pakistan.) It includes two very successful (over 1M views combined) uploads of political TV shows. I will not log in to this this accounts. So this stuff will get deleted.
So in the long run, stuff will disappear from YT.
Especially if the amount of content uploaded keeps going up, so the relative benefit of deleting old stuff is small.