So what if the player knows the game? Again, the stealth and bandana and other extras are just for fun, and with them you can take a different approach on the game. Sure, maybe it doesn't give the player 100x replay value, but they're extras.
I replayed MGS1 twice a while back and I didn't remember how fucking hard the game is even on normal (maybe I just suck???) and there are some parts that are impossible to get through without proper WEAPONS, like the part where you climb the Comm Tower for the first time, if you are short on Famas and Socom bullets and have no stun nades you are going to die.
I had 4 rations (that was max rations at that point I think) and a fuckload of Famas and Socom bullets +few nades but I still almost died before reaching the top and halfway through I was out of Famas bullets so I switched to Socom but that is completely ineffective in that part of the game so my only chance was the stun nades but I only had like 4 so that didn't get me too far.
The other time when you climb the Tower there are the guncameras on the walls, there is no way to sneak past them so you need A) Chaffs or B) Stinger missiles. I'm not sure but I think the cannons knock you on the ground so you can't advance up if they are there and they do a LOT of damage so its like 6 hits and you are dead.
Some of the bosses were harder than I remembered too but in boss fights you constantly find more ammo so its no big deal, but on the parts that I mentioned +a few others you just need those nades and ammo and the Bandana is like an answer to my prayers.
So at least in MGS1 the bonus items were really useful and you got them without being a korean. But maybe MGS1 has more replay value than the others because you can run through it in a couple of hours (maybe even less?? It was a really short game) and it only has 2 or 3 long codec convos/cutscenes and it all actually makes some sense and is pretty good and interesting.
But I'd say going through the game with extreme precision and constantly being concerned whether or not you get discovered or kill someone by accident is a really bad way to make players unlock some items because after you've done all that you most likely don't want to see the game in the rest of your life.