That's not the issue...
Maybe it's just me, but I appreciated that in Peace Walker gave me reasons to want to develop lethal weapons, especially stuff like rocket launchers and machine guns. I just feel like The Phantom Pain's reasons are pretty weak. Sure, I could be Rambo, and I am glad that people who just want to kill everyone has that option; however, in a stealth game - and this is a stealth game - I think that is a pretty mediocre reason.
The best way to explain it is in Peace Walker, you needed rocket launchers, machine guns, grenades, etc. to be able to beat probably 50% of the game - even if you are otherwise playing non-lethally. So as somebody who plays non-lethally, I understood why I needed to put in the time to get the resources and develop all those lethal weapons. And it was fun!
In The Phantom Pain, other than 3 or 4 missions, I don't need anything lethal be it rocket launchers, grenades, C4, etc. The only reason, aside from finding a new weapon part, for using lethal weapons is if you want to kill enemies. Considering the game is designed to discourage that, I think it's a weak reason.
I'm glad the weapons are there for those that just want to kill a bunch of dudes. Sometimes I just take out people too. I'm not arguing against their inclusion, I'm saying there isn't much justification for them.
At the very worst, I'm disappointed that Peace Walker justified lethal weapons that you would normally never use and had missions that were very fun where they were required, and that The Phantom Pain doesn't do it. In that respect, a PSP game outshines a current gen AAA GOTY candidate.