I can see what you're saying, but I don't think a double standard is taking place where people are more okay with MGS prequels than ones for both GoW's. With Gears, they gave us Baird to play as, someone who no one liked. At least MGS2 had its reasons for giving Raiden, Gears is all about feeling badass, something which Baird could not do like Marcus Fenix. Meanwhile God of War went from killing every God on Olympus to just fighting some weird sisters that felt a lot like weaker versions of GoW2's villains. MGS3 meanwhile is the fan favorite game despite not having Solid Snake. People are just going to accept it easier.
Truth be told though, I'm not sure if a new MGS is the right direction for Kojima period. I think he should move onto another series, keep MGS gameplay, but give a new world, new characters, even a new message. Nuclear war, a power elite, and what being a hero truly means. Good messages, but there's more of them out there and more ways to approach these ones in particular. I'll still play MGS5, but I have a hard time seeing a future where I do not consider 3 as where the series peaked.


