Edit: To clarify:
Is it even possible, financially speaking, to keep adding storage? I mean, advertisements don’t even make a lot of money, is the indefinite growth of server storage even sustainable?
Or will they do what Twitch does with old content and just delete them?
Advertising has made Alphabet one of the richest companies in the world. I assure you that it does make a lot of money.
I think OP is conflating the amount that a YT channel sees per ad vs the amount that YT would keep. These are not the same thing.
Plus, YT gets their share of every single ad seen every day. The economy of scale obviously is paying off.