It isn’t as expensive as you think to 51% attack various coins.

https://www.crypto51.app/

You only mentioned BTC but that’s just one coin. Several alt coins (with multi billion market caps) have had 51% attacks including ETC and BSV.

But anyway the original point was that it is literally wrong to say that blockchains are immutable. You didn’t address that point at all, so I guess you agree with it?