You didn't just have all the Rations you liked, though. Same with ammo. Sure they weren't exactly scarce, at least on Normal difficulty, but they were still finite consumables that you knew you were consciously wasting every time you got into conflict. Throwing away Rations and ammo willy-nilly meant having to go off the beaten path to find more, because who knows when a bossfight might pop up? Enemies only drop items for you when you hold them up or search/shake their dead/unconscious bodies, neither of which is really possible in open combat. Essentially, you're always going to come out of a combat scenario in a worse position than you were when you started.
Ground Zeroes is much more skewed to the shooter mentality. You have an essentially infinite supply of health (regen) and ammo (enemies drop it when they die). There's zero long-term penalty to being spotted. All of which serves to make the stealth gameplay less meaningful. Why bother?
I'm sorry but in MGS3 you can just run and CQC-throw everyone, in MGS2 you can just hs everybody with the tranq gun, MGS4 you have every help enabled and can blast through everyone with unlimited firepower. Peace Walker is kinda hard until you get the Sneaking Suit then it's just run and hold'em up.
And as you said "to me", I'll take it as an opinion based on your premises, but I don't think you can say that GZ is inferior to the other MGS titles
What happens when you run and CQC everyone? If you've played the game enough to know the route through every area and where all the enemies are you might get through unscathed, otherwise they get up again, trigger an Alert phase, shoot you a bunch, force you to waste a bunch of items and ammo, spend time in the Cure menu, eat some snakes, maybe lie down for a couple of minutes to regain some health. Tranqing guards in MGS2 works just fine, as long as you actually spot all the guards and don't miss your shots, because when you do you get shot a bunch and have to waste rations and end up bleeding all over the place. The Alert phase deprives you of your Soliton Radar, and the weapons and shooting controls aren't really good enough for you to easily deal with a lot of backup.
I agree that 4 was already going down the shooter route, with its millions of guns and unlimited ammo from Drebin's store, but you still had to deal with finite rations and the bizarre Psyche Gauge mechanics. I don't really have an argument for PW, since the gameplay is just so barebones and I really didn't like that game at all. Point is, even if they vary in quality of stealth mechanics, those games are all actual
stealth games, and they all use several methods of making stealth gameplay much more viable than open combat. GZ doesn't. You get your magic 'I win' mechanic with Reflex Mode, or you turn it off and hope for the best. With no equivalent mechanics to the camo index, threat ring, or vision cones and footstep loudness on the Soliton Radar, you've got no real way to quantify exactly how stealthy you're being. You never
quite know whether an enemy can see you until they've already seen you, and it can all be over in a fraction of a second with the instant Alert phase, at which point you whip out your assault rifle and play Gears of War. I absolutely think GZ is inferior to the older MGSes in terms of stealth mechanics.