There is no doubt that casinos will lose gamblers but I doubt they will lose popularity. Gambling is a big industry while sports betting a very popular market it is not the entirety of gambling, there are other types of gambling games than sports betting that gamblers likes/love to play. also, if you hypothetical scenario did happen, pretty sure those sportsbettors will find an alternative to sportbetting.
Sport bets constitute a great deal of the business casinos generate, and soccer is the most popular sport around the world and it is not even close, so you can bet that many casinos could go bankrupt because of this, as I doubt they could find a way to replace that lost income, since many gamblers only care about soccer and no other sport.
However this scenario will never happen since soccer as a sport will never be banned, not only because of its popularity but also because compared to other sports soccer is relatively free of any major controversy, so there is not much that those that could be against it could use to argue for its ban.
That is because sports part is huge and I think before gambling was invented and its games, there were already sports but we can't deny that casino games are dominating later on, especially now in the online world. That was only in terms of gambling because sports still has it's own charm. Each country has their own favourite sports, so some won't agree with you when you say soccer is more popular than others.
This makes the betting platforms survive even if soccer was removed as one of its games. Never heard that such sport is banned but rather what can be banned here is the betting platforms. That is the ones that can make them go bankrupt.