And then comes the question, what will they delete first?

Probably old and therefore maybe irrelevant content, but those old videos from over a decade ago are also mostly lower resolution and bitrate and won't free up as much space.

So once that's exhausted, what goes next?

Who will have the privilege to stay on the platform, and who won't? Or in other words, who makes YouTube the most money?

And once that has to be decided, content will be whatever YouTube wants it to be. Which I can't imagine being a good thing.