>PC port exists already
Playing the PC port is separate to seeing how the game is implemented. (At least to a close approximation.)
And since mods already exist you'd imagine the mod makers would appreciate an easier base to work on and extend anyway. Rather than trying to hack binaries and hoping it all works, especially when mixing multiple patches from different developers together.
We've seen this for plenty of other games already that had decades of mods and romhacks, but those only increased in quality and quantity when their associated game got a full source release. And developers+users can mix and match easily when sources are shared.
Not to mention if you like the game, it's valuable for its implementation to exist for study and use. People can learn from and use it as a base to extend or make new games on the same engine. We've seen this already with everything from Doom up to OoT. Getting wholly different games like Sonic Robo Blast 2, or expansion like Indigo.
Having a real source foundation is much more than just getting the ability to port the game.
Especially for the MGS games where there's so many detailed systems, it'd be nice to see if anything new is found. Or just really see how the systems interact with each other technically.