Not long ago Bitcoin and all kinds of crypto-assets were outlawed in Bolivia, a land-locked country in the middle of south America, and still I would see some people from Bolivia on the internet trying to get money out of Bitcoin and using services related to Bitcoin in order to profit from them. So even though a country could ban Bitcoin and Crypto on the paper, in reality it is very difficult for a developing country to enforce those bans and make sure people are following those laws. In the case of these countries which do not have the capabilities to enforce a ban, people will continue to use Bitcoin and altcoins but they will be more reluctant to talk about it to people they know, because of the fear they may have on authorities to find out about their holdings.
Monero and Zcash may take some more relevancy, but not as much as some people believe, though.
Developing countries don't have the resources to go after people breaking the law. Especially within cyberspace. But developed countries like the US and UK? That's another story. If by any chance they change their mind about Bitcoin and crypto in general, you can bet they'll use everything at their disposal to catch wrongdoers. Only then it will be "hell to pay".
The only way people can keep using crypto in these countries is by making use of truly-decentralized and private protocols/technologies. My guess is that people will switch to a privacy coin such as Monero or Grin, and make heavy use of DEXs. Everything will be done "under the radar", to avoid getting caught by the government. Luckily for crypto, not all of the world will turn itself against it. There will be some countries that will embrace the revolution with open hands. As long as there are crypto lobbyists fighting for the industry, we should have nothing to worry about.