So, the slow replacement preserves consciousness. Why, who knows, but it happens to us already every day, so we can take it as granted. If it didn't, we'd be someone different every day, with false memories, and while it's not impossible there's not really many places reason can take us from there.
The problem with the teleportation is not even that it's a quick replacement... it's that it's not a replacement at all. You're building a clone somewhere else, and destroying the original. You could build 50 clones at the same time on 50 planets if you wanted - and of course none of them would be you, there's zero chance you're preserved. So, you're dead, even if to the rest of the world it makes no difference.
What you're saying seems like a clear contradiction to me.
In your first paragraph, you say that we can take for granted that when an original is destroyed, having a replacement will preserve consciousness. Then, in your second paragraph, you say the opposite - that destroying an original would NOT preserve consciousness, even if there exists a replacement.
There must be some key assumption which lets you not see this as a contradiction. Maybe you believe that there is something extra-cellular which wouldn't get replicated in a teleporter?
I'm saying replacing a neuron or cell at a time within a quadrillion of them empirically leads to continued consciousness, and that assembling a quadrillion cells on a remote planet with no material connection whatsoever with the original body is a very different thing. You want to hide both cases behind the same word. I don't see why you think that's valid.
The problem with the teleportation is not even that it's a quick replacement... it's that it's not a replacement at all. You're building a clone somewhere else, and destroying the original. You could build 50 clones at the same time on 50 planets if you wanted - and of course none of them would be you, there's zero chance you're preserved. So, you're dead, even if to the rest of the world it makes no difference.
Is that clearer?