You don't need to. It would be enough to allow users to download all videos by default as a sort of local cache, have settings for max cache size and cache policy with LRU by default, and also support pinning.
Heck, doing what Google already does with Google Play updates would already be a step up.
The problem is there are privacy concerns with this type of system as it's possible for anyone to find out who has consumed particular content.
Let's say you watch new and unique youtube videos for an hour a day, or 365 hours a year. (This is a lot more than I watch, btw)
Let's say we have 5Mbps videos, or half a megabyte per second.
365 * 60 * 60 * 0.5 = 657G/year
A 1TB disk is about $50, for about $25/year to store it at home.
If you upload your videos to S3 Glacier it'll run around $8/year (per year, so after a few years this is more expensive than buying drives).
For comparison, YT premium is $144/year.
I only watch about 10 minutes of unique youtube videos a day on average, so I'm looking at around $1/year cost if I want to archive every single youtube video I've ever seen. It's pretty affordable.
I wonder for how many users this is true. I personally watch several hours a day of youtube because it's mostly on in the background instead of music. Lots of infotainment channels. I'm pretty sure on average it's like 20mins per day can't find the source though.
I was just thinking about this problem from a different angle.
What if everyone, or at least a bunch of willing souls, ran the following script on their on pc's:
1. YouTube-dlc every video you ever watch automatically in the background and save to NEW_VID directory
2. make a checksum of every file that comes into NEW_VID
3. Check by using the checksum if the file already exists on open platforms like https://joinpeertube.org, https://odysee.com or https://d.tube
4. If not, upload the file.
5. Delete the file.
I was thinking about this a few months ago and wish there was some sort of background app that could keep and eye on my YT accounts likes and archive them.