Everything comes down to social consensus at the end of the day. But for 99.9999% of transactions you can count on immutability.
Like for example in the world of finance, where you have to be able to backtrack.
Meaning such 'contracts' are least useful for things like, well, 'currency' and 'stores of value' ... hmm ...
Yes, a blockchain gives you an (ideally) immutable foundation. No, that doesn't mean that every transaction that invokes a smart contract has to be immutable. If a smart contract for a particular use case needs to have the ability to "backtrack", so it can, there's nothing stopping it.
Needs backtrack, code it in, then everyone knows how the rules work and there isn't any side deals or exceptions.
Cheers!
I think the bigger issue is that the system is somewhat arbitrarily controlled by the large players. That could work out well in some cases (funds hacked are returned) but it could also be less optimal (e.g. you're thrown on some list and all of a sudden your transactions are not valid). We've already seen hackers and obviously malicious actors dinged, which is good. But this opens up an avenue for things like forcing participants to go through regular banking protocols that starts to affect more and more people (e.g. political dissidents). By then you just recreated the modern financial system with all its flaws and gatekeepers, except its less efficient.