This is the correct answer. The cartelization of bitcoin mining has made it more centralized as opposed to before when anyone can enter the network and mine with their CPU. The rising difficulty is making it more and more a competitive area for miners. You need specialized hardware if you want to join the mining. With that idea in mind who is to say if collusion between miners is not possible today? The only thing that is stopping them is their incentive will be reduced in value if they are caught colluding.

The number of nodes are also going down because it is getting more expensive to maintain one because of the blockchain's size. There should be a mechanism to incentivize people who want to maintain bitcoin nodes.