Can't youtube just ban all the channels that decide to join the union?
Well, as long as they don't actively promote the union in Youtube videos, I don't think they would really have a ground for banning them. As they are independent workers, what happens outside of Youtube is probably none of Youtube's business, as long as it's not illegal per se (such as false claims/false information/etc.)
But if they do, that could easily pass as disloyal to Youtube, which then could ban them I suppose.
I think that might go to some higher court then, as the connection of unionizing and disloyalty would be as much an imputation/libel as it would mean they handle creators like employees. Probably won´t happen unless legally required to treat unions as terror organizations.
In itself the problem is less that YT rewards creators for content the way they do (they somehow needed, otherwise there would be no way to sort out the copyright violations that threatened the platform, by setting it directly on a monetary base and then redirecting the money to the source), the assumption that you can depend most of your income on youtube was made by the creators in the first place, based on the experience they made so far.
The hidden employee status might be too big of a stretch in a freelancing scenario, but i think a contract that involves being paid at will of the other party is a legal problem of some sort, after you invested effort into it. There is some ambiguity, as the content might just have no views or be of low value, but high view count/watch time and demonetization without a reason given kind of break the contract, and willful setting of the price does not help either. The value is not the only parameter, it could also be the measurement of watchtime (not independently measured).
Unions don´t care about freelancers, they usually work for the contract details as offered. It just happens that this falls in the same time in which others try to pay for labor by making it look like freelancing, although the laborer is doing full time for them under horrible conditions. This would be a huge loophole which even the government is not interested to grow big (loss of taxes, voters, undermining of workers rights and laws), so the unions get involved.
Imagine the extreme situation in which a channel would consist almost exclusively of videos bashing Youtube. Not sure it would last for very long.
They don´t have to delete it (that might get them into problems with free speech, depending on country), but they can set the value of it and not recommend it to viewers.