Even if poker is a PvP kind of game the casino still has expenses to cover and since they are giving a service and they are using some of their resources to keep the poker tables online then it is natural they take a rake, and as you say this rake is lower than what we see at physical casinos, however I can understand why some may find difficult to profit from it as the number of poker players on those websites is limited and many are decent players so unless you are way above them in terms of skill then you are not getting any profits against them.
I think that you can find online casino or poker room with players who`s skill will be at least not higher then yours own. But the most interesting game is when you`re winning powerful opponents as for me. In such wins you usually getting good profit and good moon. In easy games you can get profit, but mostly it will be less then your normal profit, and surely it will not be fun.
The trouble with these smaller and niche poker rooms (e.g. cryptocurrency only) is that they tend to be dominated by sharks. When you're on somewhere like pokerstars there are thousands, if not tends of thousands of players, and lots of new fish sign up each day to leak cash. This keeps the sharks fed while the fish either learn the game over time or drop off. However if you are looking at somewhere like swc where there might be 150 players online, who are spread across different buy-in tables, you'll find that they often know every single player there and how they rank against them. New fish are rare - so they get targeted and wiped out very quickly, which leaves you with a constant low amount of players. It's hard to see a crypto poker site doing well unless we have much wider adoption or a single site throws so much money into advertising they can reach a "critical mass" which draws in a sustainable amount of players (like having at least 1,000 online at any one time)
Exactly, poker is a zero sum game, if we want to earn money then someone else at our table needs to lose money, if everyone is a good player and we have a similar level of skill then over the long term the players are just bouncing money around and no one earns anything except for the house with their rake, this means that new players or players with low skill are needed but if their number is low then sooner or later they will give up and you will be back at having the problem of only playing against skilled players, so what is needed is for poker with cryptocurrencies to become way more popular so the influx of new players is constant.