However, it is also clear that the Bitcoin price cannot more than double every four years for all time. This means that fees are becoming increasingly important for miners.
Since price is already going a lot higher than the reward is reducing, it is clear that fees are not becoming important yet. There is no indication that they will become important in the near future either. That's the problem with this type of arguments, people are speculating about something that hasn't happened and we have no evidence of how it will happen.
This contradicts the decentralized structure of Bitcoin.
I do not get it, why do you believe Mining becoming increasingly important for Miners will lead to a contradiction in the Decentralized structure of Bitcoin?
I didn't write that. I wrote that
fees are becoming increasingly important for miners. The increase in fees means less decentralization because the miners become more dependent on the users.
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That does not contradict decentralization.
If the fees exceed the reward, is it possible to add a proof-of-stake layer for validation?
No because PoS is fundamentally flawed and is a terrible protocol. And I personally believe that if we wanted to change the mining algorithm in any way, we should find a new and proper algorithm to replace it with not something broken just because it looks like the easiest solution.