The immutable ethos in Bitcoin has led to very slow changes at the protocol level and no hard forks.

How so? Are you saying that ETH will develop quicker because each time they screw up they will just hard-fork? That's nonsense.

You seem to be confusing the immutability issue with the governance issue. What stalls Bitcoin's development is the unresolved governance issue, and sticking to the immutability principle affects BTC price but has nothing to do with the speed of developement.