24% Off total YT income means a lot for big YouTubers.
It isn't off total YT earnings, just off the earnings from viewers living in the USA and TBH, for the vast majority of us who do not live in the USA, that will not make a huge impact.... I just did some quick maths and using my channel as an example, and rounding up to make my brain not hurt lol
in the lifetime of my channel if I had to pay tax on all my YT earnings from USA views it would be under £30 to cover the last 4 years.
The larger channels would be hit harder like you say, but it would only be on future views.
As a side question, I wonder how YT will actually calculate this, as some of those viewers will be running ad blockers, others will not be in the USA but running a VPN suggesting they are, most will not watch the whole video so any mid roll and end roll adverts may not have been seen, so the actual revenue earned over a tax year will not necessarily be a direct split between the percentage of views from the USA