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.