It's actually a pretty big topic in legit altcoin projects, which try to implement Satoshi's original '1 CPU, 1 vote' idea as stated in the whitepaper:
This is a simplification of explanation not a promise. It is not like ASICs were invented after bitcoin invention. They existed ever since 1967. Not to mention that having ASICs or even a ASIC farm doesn't centralized mining as long as these farms are distributed around the world which they are.
What centralized mining (even if you can only mine it using CPU) is when majority of hashrate is controlled by a handful of people. This has never happened in bitcoin (ignoring day 1 of course when it was only Satoshi and a couple of others mining blocks).