There's more to being a successful human than straight A's in school, and for some people the A's are easy.
I think if the parent thinks the agreement they made was stupid and is now a detriment to their child's well-being, they should edit it.
Acknowledge having made a mistake. Don't pretend you were right from the start. That's an important part of the learning process, and your kid needs to know it's safe to make mistakes and adjust course later, that you're not stuck with something just because of one decision in the past.
Be clear on what concerns you and why (especially be sure that it's a concern for the child's well-being and not some need of your own that you're trying to meet by changing your kid) and have a conversation with this young person about it.
Eh, like many things in life, it’s not that simple, but the reality of it is that YouTube plays a role within a clear framework intended to bring wider sense of balance their life (BSA scouts, sports, literature I choose every now and then, etc). I wouldn’t quite phrase YouTube as being straight up detrimental to their well-being, it’s more that I personally don’t enjoy the baggage that comes with the benefits. So like Natty Lite, 1-2 are probably fine (even if in poor taste) but beyond that it’s a problem.