You're missing one of the major options there, which is new coins! Yes, we know they're sometimes difficult to implement, or there won't be much of a player base at first, but don't you know that Bitcoin is as successful today because it was first accepted on casinos? It takes time for success, and accepting something before it's due will actually yield benefits for everyone as it increases the utility of the crypto, and hence its value.
You mean that online casinos are promoting cryptocurrency. Maybe you are right but I don't think so. I think that you should not use your cryptocurrencyin online gambling, because it is a business and if you use your cryptocurrency and online gambling it means that you are using your business which is not good. If you lose your money that's ok but if you use your business it's horrible.
Anyone can actually use their cryptocurrency however they feel like as long as they know the implication of what they are doing and they are matured enough to take the responsibility for their actions. However, in a way I would rather say cryptocurrency has actually promoted online gambling than online gambling really promoted cryptocurrency.
Looking at it from this perspective, so many people who are gambling online now got a glimpse of gambling after looking for a way to at least increase their cryptocurrency which unfortunately then just ended up choosing the wrong means.