This is why I download and archive locally things I enjoy repeatedly and find significant. If I remember something I once loved or laughed hysterically at back in the old days, I use yt-dlp to download it because I know someday it'll get erased and a shrinking minority of people will even remember what it was.
To me, it's about personal culture. There are videos and other online content that became an institution in my young brain, but inevitably faded into irrelevance, eventually disappearing entirely in some cases. If I remember a piece of media but find that it's been lost for good, it's like I've lost a small part of myself.
In any case, I definitely recommend creating a personal archive. I agree that the Internet Archive is an amazing thing and that we are blessed to have it, but clearly not everything has an obvious place on it and I think it's only a matter of time before the Cathedral, if you will, recognizes its existence and subverts or destroys it.
I could not have phrased it more perfectly myself. Those things are a part of us. Back in the day your preserved memories were a journal, a book, a photograph, a videotape. Now they live on the cloud and all it takes to lose part of it is for a creator to decide that content doesn't suit their brand anymore and erase it. Just going through your Youtube Liked Videos list and seeing how many are no longer available should be enough to send a cold chill down your spine.
If I like it, I save it. There are some things I suspect I may have the only existing copy of. I wish someone would come up with a site archiver like HTTRACK but built for archival of modern websites that are full of content not hosted directly on the site's server.
The search feature also seems to return less in general, and has become geared towards suggesting content below the fold. You can get it to keep returning more that's related to the search query by just continuing to scroll, but it runs out remarkably quickly.
So yeah, if you like it, download it. I don't care what anyone says about copyright. Unless it's something you know will be around, it can disappear for good.
I'd love a multi-billion line spreadsheet that just contains videoIDs, titles, channel names, and descriptions from all of YouTube.