See I don't agree with that at all.
Sure the shooting and stealth gameplay of 5 was good, but what surrounded it wasn't. The open-world felt lame and more like filler to get to where missions were happening. After a few hours, traversal felt more tedious than anything. I hate the mission select structure and the general Peace Walker 2 type stuff. Complete with a pretty boring and uneventful story and you have a game that felt like it had no focus and no momentum. There are no amazing moments or boss battles, no instances that stay with me like so many moments of MGS1-3. It just felt like such a soulless game.
And I especially hate PW. As a single-player game, it's an absolute grind fest. Gone are MGS personal boss battles, instead now you fight a tank. A tank that can take tons of damage and rockets to kill. A tank that's not challenging per se, but one you'd shoot your 5 rockets, go hide, call for supplies, and then do the cycle over again. Of course, I guess you can develop better weapons to level up your rocket launcher. So that means you'll just repeating missions over and over again to futton more soldiers. But the missions are super boring. Small spaces complete with stealth that can just be you in a corner and shooting people. Which is even easier to do when you're not playing on the PSP and aiming is nothing. The grinding comes complete with a retcon-filled story that takes more shits on MGS3.
I dislike these games. They don't have anything I liked about MGS 1-3. It blows my mind that they never made a MGS game on the PSP that played like the trilogy. With one analog stick, it would have fit the system so well.