Player choice and The End.

[humanoid]

The body inside is dead so it doesn't need oxygen or thermoregulation, so the key question is the impact of pressure.

Thats what I was thinking as well, however clearly the suit is supposed to protect Simon-3 (even if its just a dead body containing a cortex chip), otherwise we wouldn't need to get it in the first place.

Not to mention that Simon-2 literally dies due to high pressure when you jump down the Abyss, it even shows a little cutscene for that (at Omicron).