These channels are not ordinary channels. The channel owner has to work hard to achieve the number of subscribers in the channels. You have to work hard year after year to make a channel popular. No one will sell their own channel.
There is both possibility here. You can't do nothing if any of these channel owner sell his channel for a good amount of money. Its because only scammers know how much they can earn from this massive amount subscribers and if i am not wrong then their offer wasn’t a little amount.
So I think these channels have been hacked. Hacking and promoting such scams.
Yeah it can be but why didn’t the real owner reported to youtube authority yet. Personally i prefer to submit reports after seeing anything like this and IMO it's the most fastest way to stop this type of video streaming.
Good news is that someone have already reported all three videos and youtube authority removed all from their end.
@ScamViruS you may forgot to use the correct spelling of "Cardano"