Reputable casino with bankroll investment feature wont scam people, lets say they are going to close the service then they will inform their investors so there will be a time given for the investors to take their invested money.
Of course the worse case may also happen, so when it comes investing in casino's bankroll, people should always invest what they can afford to lose only.
Not only that casino may turn into scam later but there is also a chance of casino getting hacked and the bankroll stolen.
This investment can be said as a good long term investment but the risk is always there.
You have a point, as we have a casino on this forum that did the exact thing you explained. However, this is an investment, and there's nothing wrong with playing safe by going for a reputable casino we have on this forum, which offers such services, because it will be easy to be aware of their new developments, etc.