So let's ask the uncomfortable question: Could Bitcoin face the same? And if so, would exchanges freeze BTC deposits, grinding everything to a halt?
The nature of Proof of Work is in a way that 51% attack is a real attack vector but it depends on a big "if". IF one entity can gain control of such a large hashrate they can theoretically carry out such an attack.
2 more things, you can't really compare any altcoins with bitcoin.
Also this already happened a long time ago in bitcoin as well. The mining pool was called GHash.io back in 2014 and it briefly gained more than 50% of the total hashrate (for real) and nothing was halted and nothing broke. The miners left that pool
(switched to other pools) and its hashrate came down.
• Mining centralization is real. Foundry + Antpool already control ~50% of Bitcoin's hashrate. It's just expensive to attack - not impossible.
I haven't verified how much hashrate these pools control to verify your 50% claim but you are forgetting that FoundryUSA is a US based mining pool while Antpool is a Chinese based mining pool! That is not called centralization.
In fact this is the exact opposite since these pools are located in completely different jurisdictions.
A well-funded adversary (cough nation-state cough) could try.
And what would they gain by spending such a massive amount of their taxpayers money? Specially while they can "fight" bitcoin using much cheaper tactics like simply banning it!
• Would Proof-of-Stake fix this? Or just replace mining barons with staking cartels?
PoS is a broken algorithm that died as soon as it was invented (ie. ~2012). The only reason why it is still used by shitcoins is that it is a cheap way of getting a coin that has lost the community to stay alive.
• Should exchanges decide network integrity? Kraken just set a precedent.
They should not and they are not.