China has been a big player in mining, but not in trading or adoption. Meanwhile the US is a home to the biggest Bitcoin exchanges and other companies, and a lot of trading volume is coming from it. EU is also a very important market. If these two entities ban crypto trading, it would be like the asteroid that killed the dinosaurs. Most coins will die, Bitcoin will survive but will become very small in all senses - price, popularity, community, network.
This is why it's very hard to ban crypto on that country since mostly the market which is the people will have a hard time coping up the losses. I was even shocked years ago when they announced that China is prohibiting mining coz as far as I know they were amongst the first to recognized crypto and mostly launched campaigns to promote it and then suddenly they just declared out of the blue.
No country can completely ban crypto as there will still be several ways for people to invest in bitcoin. But I agree with what hatshepsut93 said if these two powers ban cryptocurrencies, it will be a big blow to the market. We will not die completely, but it will be tough for us to survive, and it will not be as easy to make a profit as we are now.
I don't expect any of these two countries to have a strong crackdown on the crypto industry. For me, China's bitcoin ban is temporary, and they will reopen soon because I believe they won't want to be left behind from the rest of the world.