There are still online business who didn't excel that much because their business are not that famous enough or they don't have a good service. Then there are new comers who boomed. I don't think moving online is costly especially when the online business became a hit because there are lots of cheap website offers are then coming out. The good and bad effect of the pandemic, are not only applicable to the business owners but also to the customers as well. Many lost their jobs while some got lucky to have a new career online. Online gambling provides some comfort but at the same time it can make the person to get addicted easily.
The reason why some casino businesses didn't gain significant success is probably because of lack of promotion, a casino business is about 70% dependent on promotions, and the remaining 30% goes into service, games, offers, and all other things. This means that a casino can never get successful if they don't have proper promotions running all around the internet and they need a team of experts for that to happen.
And it actually isn't that cheap to move from a land-based to an online platform because the development costs are skyrocketing these days and to have a good platform with proper marketing and everything, one will need to spend quite a lot of money, it will cost more if they go for developing own games.