I guarantee the longer then expected development time is due to them realizing they had to rebalance the entire game around the new camera and controls.
Enemy a.i., line of sight, health, etc. I’m no game developer and this is just speculation but it makes sense.
Edit: I’m an average skilled MGS player but it’s my favorite series.
Returning fans like me (& probably even new players) would surely wipe the floor with this game were they to hand over the same challenges that the original game had while equipping us with the player agency of MGSV.
I’m not the first one to bring this up but I don’t think it can be overstated. If this is the case, they should be better prepared to remake even older games in the series like MGS & MGS2.