I took the liberty to play RE5 to see what went wrong. A lot went wrong. The biggest problem was it wasn't a RE game anymore. If it wasn't for the characters, it would have been some sort of generic third person shooter with a functional cover system.
The worse offense was the multiple compromises done to the game in order to pry bar in the coop system. Starting with the levels themselves being no more than a bunch of strung together kill rooms moving you from one area to the next without any flow or any real motivation as to why your moving forward. The levels are so streamlined, there is no real point exploring. Nothing about the levels are atmospheric with barely any meaningful lighting applied. Way too many pointless cutscenes with no purpose pointing to a much better gameplay location.
A "high speed" but a very limited inventory system now takes place of the Tetris suitcase. It is increasingly aggravated by an AI partner that is ammo limited as you are without any human skill or consideration regarding ammo, weapon usage or proper shot placement with only 2 modes. One mode is shoot to wound while sticking close to you in order to "Cover you" or attack which switches the AI to a head shooting machine that expends ammo regardless of effectiveness.
Worse was that they cast Chris as the main protagonist and quite frankly, he is a meathead whose only expression is "Grizzled Robot". Sheva has a laughable accent that varies between English, touch of Australian, and some actual, but wholly generic African accent, but it's mostly English.
Playing on Veteran difficulty is impossible on first time out despite been available out the game and would require one to grind levels for money to upgrade guns and to skim as much ammo as possible, but eventually you will run out of ammo and the game becomes impossible. The game gives you a gun with unlimited ammo to compensate for how broken the game is on completion of normal difficulty.
RE:R avoids a lot of the traps RE5 fell into by sparating out the multiplayer component. However it does suffer somewhat by the levels dual use nature. The combat dynamic is very different to compensate for online lag and I much prefer RE4/5 combat.
In the end there hasn't been a true sequel to RE4 which did everything right. RE:R is better than RE5, but I really want a proper sequel to RE4.