But that strains credulity.
Sure, that's part of the issue, really...
If the road from Winterhaven to Hammerfast is now invested with Titans, the road is now impassable. Or did the peasantry level up too?
... but it's the angle you have to squint at the treadmill from to see it that way.
In the context of 4e, the game is not intended to support travelling back and forth between Hammerfast and Winterhaven dealing with encounters on the road.
Not from first through 20th level, certainly, not in the context of any edition, even 5th would 'strain credulity' a bit if you found enough kobolds to get enough exp to get to 20th level just walking back and forth between two towns.
But, if you did happen to drop by your old stomping grounds 10 or 20 levels later... what would you find? If you're able to see the treadmill as a travesty of a mockery of a sham, you probably found a Githyank invasion or 11th level kobolds 10 levels later, and Epic-level Kobolds ("just how many Sons did Kurtulmak father in the Nentir Vale, anyway?") or a clan of Titans trying to bring back a Primordial 20 levels later.
This is clear from the descriptions of the tiers of play in the PHB and DMG; as well as in the flavour around paragon paths and epic destinies (eg a Knight Commander/Legendary Sovereign, or a Warpriest/Demigod, is not meant to be framed into random encounters on the road).
Sure. I mean, unless they're meeting the Buddha on the road or something.

But that'd be Fate.
And meeting kobolds on the road at heroic tier, and then titans in the Elemental Chaos as you try and reforge creation during epic tier, isn't really a problem, is it? (And at paragon tier, the PCs should be dispatching
whole phalanxes of hobgoblins.)
No, of course not... but if you did happen to decide to go slumming and wipe out a kobold tribe back in the Nentir Vale on your home dimension, you wouldn't expect Titans to have displaced them or the Kobolds to have all gained levels at the same pace you had. Maybe the DM would have you fight 'swarms' of kobolds - the whole tribe at once - to make a playable encounter of it, or maybe he'd just hand-wave it and give you no experience, but the expectation that if you take a break from cruising the Elemental Chaos and beating down uppity Titans, you've come a long way from fighting for you lives against kobolds led by a berserk goblin, and it'll show.
That is, unless you were playing the game a certain way... and if you were playing it that certain way, from that PoV you'd see the 'advancement' of the treadmill as a hollow thing, since you couldn't ever seem to /find/ anything significantly weaker than yourselves. Or, for that matter, to take it from the other direction, anything all that much more powerful than you (a solo a few levels above you, sure, would be much more powerful than any one of you, individually).
This "story escalation" that is inherent to the 4e design may or may not be a good thing - some people want the game to stay prosaic, in a certain respect, even as the PCs gain levels - but I think it's a reasonably apparent thing.
It's implied in levels, really. Back in the olden days, you started out a magician's apprentice or a veteran of one battle, and ended a Feudal Lord in his castle or reclusive Wizard in his tower - and, if you didn't end it there, could go dungeon-crawling in the Abyss and kill Orc
us, instead of dungeon-crawling in a dungeon and kill orcs. Once monsters got a clear 1:1 corresponding CR/EL, it was inevitable that someone would fall into the trap of just matching the party against same-level monsters all the time. The futility of that might be masked if you had increasing imbalances within the party as you leveled (though there's some obvious futility in that, too), but when you're all neatly balanced, you can more easily 'see the wires.'
You could keep things 'prosaic' by just ending the campaign sooner. Just play to name level. Just play E6. Just play through Heroic Tier.