My honest view is that even if the majority if the hashrate happens to be in the US it won’t harm bitcoin’s decentralization.
It will hurt decentralization. The only way it won't is if the percentage of hashrate in US doesn't go above 5% (roughly the same as US population/world population).
But that is not the biggest problem. The biggest problem is that if "majority" of hashrate were inside US, it would significantly damage one of the main Bitcoin Principles called censorship resistance since as I said the US laws enforces censorship on miners.
If bitcoin is going to get censored and destroyed like that, maybe, it never deserved to exist in the first place.
If that’s a real threat, we are already facing it.
The data I read says more than 30% of the total hash rate is in the US which is the biggest piece already. I think there is room for another 10% at least.
Let’s say the worst has happened the US gov started to censor lots of transactions, the miners will go to some other country and abandon their businesses in the US. If all govs fight btc like that, see my first sentence.