As mentioned, PCs past 5th level or so have gone outside the normal human range. By 10th level, we're not talking Conan - we're talking Hercules.
But that's been said. What hasn't though, is that there's a risk to adding any kind of "compensating" rules to make groups of mooks more dangerous - it significantly inflates the power of armies, to the point where even a small town barely ever needs heroes.
For instance - at 10th level, each character is CR 10. Interestingly, an 11-headed Hydra is also CR 10. Yes, that in fact means each character is as dangerous as an 11-headed Hydra (not just coincidence I mentioned Hercules). Now let's say that hydra is seen headed toward a town. Which seems a more appropriate response by the mayor?
"A great monster approaches, and we have little time to defend. Barricade everyone inside the town hall, get every armed man ready, and send the fastest rider to summon the royal knights. If only there were some heroes to aid us!"
"Drat, another nuisance monster. Sheriff, round up a dozen or so men and go slay it."
And if a Hydra doesn't seem impressive enough, consider a Bebelith, Fire Giant, or other monster of that level instead - there's lots to pick from, none of which seem easily defeated by a small squad of guardsmen.
If a dozen guards with crossbows can do that, then imagine what an army could do! Forget dragons terrorizing a kingdom - they'd have trouble terrorizing a moderate sized city. Ancient forest of evil? Screw that, we've got crossbows! March through it, wipe out the ur-demons there, and get some logging going.