The 5G limit is going to be decided in concert with the minimum altitude and an upper speed limit.

GLOC would be most common at the merge, where they'll be at corner speed (the speed that allows them to get the tightest turning radius whilst bleeding the least amount of energy) and need to instantaneously load to 9Gs.

So from an engineering perspective, I'm sure they massaged the G limit, minimum recovery altitude and maximum likely speed into the most optimal set of parameters, with 5G offering a good trade-off between minimum recovery altitude and pilot comfort.

The planes themselves are G-rated to 9Gs, which is entirely a flight control system limit, because the bag of bones in the front seat tends not to do so well over 9Gs for sustained periods -- but the actual airframe will be able to sustain much higher before structural failures will start occurring.

Within the cockpit, on the left side, there's a CAT switch that's used to switch between CAT-I (lighter loadouts, such as A-A) which permits more G-intensive maneuvering, and CAT-III (external tanks or A-G ordinance).

As far as sustained loads higher than 5G's, I could see the potential to kill someone, but I would also guess that the more immediate threat after G-LOC is probably the ground.