You are talking about Ponzi scheme here, but how can you prove it? I mean why do you think that all services work this way?
It's actually very simple.
The largest cloud miner that's actually mining is Bitmain's HashNest. Everyone can see what they generate because their mining addresses are public. Based on the coins they actually mint, there is zero indication of distribution amongst the tens of thousands of individual cloud contract holders. In other words, users are getting paid with money coming from a different source, namely, previous investors, which means that it is a scheme.
The sea full of other cloud miners don't have the balls to release their mining addresses, because they very likely aren't mining at all, or only on a very small scale.
Everything above is very easy to fact check by people having active cloud mining contracts; withdraw your coins and you'll be able to see that it doesn't come from mining addresses.