Let's talk about this.
We appear to be talking about different things. You're talking about a port of a game from a previous generation to another. What I'm talking about is them using the very SAME map from RDR2, then add the motion captures and the voice overs from the original RDR to the RDR2 engine. The only thing they'd really have to do is re-create the specific characters from RDR that Marston interacts with, and perhaps make a few tweaks to the towns to update them to the 1911 timeline.
But I agree with you - attempting a PORT of the original RDR to the current console generation would be a calamity. But again, since they have the entire map updated and recreated, there'd be no need to port it.
Let us not forget the epic round-the-clock scramble the poor folks at R* went through to hit their release deadline of RDR2. Had they finished early and had extra time, I'm sure that conversation (release an updated RDR with the new map before releasing RDR2) would have taken place. But up until zero hour before release, they were 100%+ swamped. There would have been no time for such a thing. That's why I think now is the right time to think about it.