But there are people who are constantly downloading the let's plays of those youtubers, so there will nearly alway be a copy of a video if it isn't on youtube and it might end up on archive.org later on
Are there really? I do watch a lot of let's plays and sometimes download them in bulk, but I don't keep them around for long. 1080p60 takes a lot of space and recently a lot are even in 4k.
You just need to visit r/datahoarders to verify this claim. No, I don't understand it either, but it is definitely something people do.
Are there any concerted efforts to archive these videos currently?