How will bitcoin stay decentralized when mining becomes extremely expensive? My understanding of bitcoin is that the miners are the ones keeping bitcoin decentralized. What happens when only the richest of people can afford mining?
This is a valid concern and I think nobody can give you a clear answer here. We'll just have to wait how it evolves and how people react; it is an unclear future.

There are some ideas about changing the proof-of-work algorithm to make mining pools not that profitable, but the effects of such implementations could be various.

I highly recommend this article from Vitalik Buterin, where he walks through the history of Bitcoin mining, explains problems and solutions encountered in the past and describes the situation today. He also dares to predict the future development and provides some ideas to keep the mining decentralized.

It seems there's still a long path in the future of Bitcoin development.