We should immediately withdraw it from online casinos before they think of a way to hold it and then ask several questions, you knows, ways that you might feel that they don't want to pay or delay it if it is really that big amount.
For investment, yeah, for sure as crypto enthusiast, we have heard this before as crypto in general is all about investment and waiting and holding and then selling at the right time. So it could be crypto at best for us, or real state for good. Maybe we can go back to gambling again, but be sure to control ourselves and not let that big win or the money that we have won, returning it back to that online casinos.
I think only a few who can think like this and they can usually think of it if the casino they are playing with is still new to them. For the most gambler, the reason why they are only rushing to withdraw is because they are afraid that they might lose it all by getting tempted of playing again and again.
There are still a possibility that we can redeposit especially if the money is still there, so many are literally spending it on things, food, paying the bills, etc... For a legit casino, I don't think they will question you for having a lot of funds in your account but they will be happy about it in fact because they see it as a form of support.
There might still be a casino who are also happy when their customers successfully withdraw their money because that can show of being a responsible gambler.
Another reason on why a gambler can hold money on his account is like you said if they are investing because there are also casinos who have this feature. It can be a good addition to our other investments because casinos now are still getting a lot of demand. Crypto in general is actually all about currency or payment and them being used as an asset for investment can only come in second.