GTA5 was probably too costly, hence why the other projects like LA Noire were done first, to get a better idea of viability.
I think you are overreaching on RDR. I doubt that would have done enough to warrant the porting costs and time, especially when the sequel is never hitting the Switch.
Considering that many outlets predict that the sequel is going to become the best selling game of the year, plus having a ton of marketing on the game already, the original Red Dead would've sold more than a million on the Switch easily. Maybe when T2 decides to make the remaster for current consoles, though considering the PS5/NeXtBox are somewhat closer, they might wait for them, which would leave the Switch version in an akward position.
You know it deserves the GOTY.