I agree with you there. Nowadays the identity points to a single thing without any notion of time, in the perfect world it would point to a set of things with their respective dates (and the database would have operations to deal with that).
If you want to get philosophical you can argue that in the real world everything mutates, even the identities. Heck, you can even say that the identity comes out of the value (the river is just on your mind, there's only water flowing). But in our digital/fuzzyless/mechanic computing models those abstractions are just unrealistic.
Identity, or objectness, is a distinct concept from being a value that is defined purely by its form. So in that sense, identity is immutable because it is defined that way, it isn't a property that falls out of the universe. At least, its been that way since the Greek philosophy days.
A ball is an object, it has state, if its state changes, we can recognize it as being the same ball. A point/position is a value. It doesn't make sense for it to have state, a position with a different value is simply a different position. Getting around the arrow of time is something else entirely.
If you want to get philosophical you can argue that in the real world everything mutates, even the identities. Heck, you can even say that the identity comes out of the value (the river is just on your mind, there's only water flowing). But in our digital/fuzzyless/mechanic computing models those abstractions are just unrealistic.