If RDR received the same reaction as IV there would be no RDR 2.
How do you even know that? You act like GTA IV was a universally sh*t canned game like Ride To Hell or something that had no chance of ever producing another sequel.
Because if a new franchise gets a mixed reaction there is little motivation to do another one. I know it is technically a sequel but very few people know about Red Dead Revolver. GTA was already a beloved franchise before IV came out so they tried again with a sequel.
Watch Dogs springs to mind. Many people didn't like the first game, but they were still motivated enough to do a sequel.
Whether people loved GTA IV's new direction or not it still doesn't change the fact in R*'s eyes it was a financial and critical success. Even the beloved 3D era games still had their critics. GTA IV might be a divisive game, but there's no way to know had Redemption got the "Um..uh...oh that's actually quite cool" kind reception GTA IV got it wouldn't produce another game.
As others have said each GTA game strides to be different from the last so with that you're likely going to get a bigger range of opinions/feelings among the fanbase than the considerably smaller Red Dead one.
Sticking to the idea of this thread even if R* play it safe and stick to the formula that worked for Redemption it doesn't guarantee this game won't be a disappointment. R* have to be careful here as they don't want to play it too safe where barely anything's changed, but not tinker around too much where it loses its charm from Redemption.
So to me there's still an element of potential disappointment if R* assume they can just copy and paste everything that worked in 2010 and apply today. GTAO''s deadly poison has the potential to really f*ck things up if R* get it wrong by putting it in the game.