At what level would/will/have PCs in your campaign world "topped out" of the challenges available in the Material Plane world?
30
What I mean is, at what level will they stop finding challenges without going off plane?
Actually, 30 is the cap for off-plane too. By 30, they're damn near demi-gods.
And if your world has high-level challenges (10+, 15+, 20+, 30+, etc.) what has kept these challenges from totally wrecking the world before the PCs were around?
Nothing. The world has been wrecked several times, twice while the PCs were unable to do much except survive.
The things that challenge a 20th-level group could completely wipe out most cities smaller than metropolises -- why haven't they?
They have. The ones that remain do so because of (1) luck and (2) the many 20th level NPCs who give their lives to protect them.
Are there any things in the world that "always have been, always will be"? Like a guardian that has never been passed (and could never be passed)?
Yes.
If you have such a thing, do you ever expect the PCs to level up to where they can defeat the thing? If so, why hasn't some NPCs ever leveled up enough to kill it?
No. Things that "always have been, always will be" must be unkillable or must self-incarnate if destroyed.
Do PCs ever retire? Are there always "bigger fish" in the campaign world?
If a player wants his PC to retire, he does so. (I guess I don't understand this one.)
Since I'm not interested in running games above level 30, but the CRs for critters in my world go up to 57, the answer to that last one is "pretty much".
(Commoners? NPCs?)
Extinct. They all got eaten by demons a thousand years ago. Humanity is smaller in numbers than in our world, but each human is individually powerful. Average NPC level is 10; max NPC level is 20. Each city has 5-30 NPCs of 20th level. The world has around 40 Archmagi (20th level Wizards or Sorcerers); their names are fairly well known.
(Why don't these 20th level NPCs do everything?)
In addition to possibly having some NPC class levels, NPCs lack one thing all PCs seem to have: a deathwish. NPCs are mostly sensible folks cowards who are delighted to pay someone else to raid the goblin lair, while the NPCs merely defend their own homes, and only fight as a last resort.
Cheers, -- N