I just don't want your regular 1st level monsters to be so weak as to never be a threat to a high level PC. Sure, the high level PCs should mop the floor with them, but there should still be the threat of serious damage, even death. Even the mightiest of heroes is still vulnerable to the perfectly placed crossbow bolt or dagger in the back (or, bullet to the head if you're using firearms)
Bounded accuracy definitely helps with this. ACs should max out around 20 (right now they max out around 25 but I think that will be corrected with some stacking rules). Which means anything can hit you, and even many first level creatures can hit you.
For example, Kobolds have pack tactics. Which means a normal Kobold can get up to a +6 in large numbers, and a Kobold Dragonshield can get up to a +8 in large numbers. For reference, the Balrog attacks at a +8, and a Red Dragon at a +6 or +7 depending on the attack. So those Kobolds in a swarm can definitely hit high level characters.
Of course, the party will have a lot more hit points by then. And they will one-hit those Kobolds (though they might miss a few of the Dragonshields, who can impose disadvantage). But yeah, I suspect in large numbers those Kobolds will be a serious threat to a higher level party.
By way of a real example, my third level party were seriously damaged by a troop of skeletons (wandering monsters), which are weak first level creatures. My PCs were caught during a short rest, which is a one hour rest. Their spells were depleted, their turning attempts were depleted, and their arrows were not very effective because the skeletons are resistant to piercing weapons. Only some exceptionally good tactics by the players, funneling their foes down a narrow bridge and then changing out their two front liners regularly to spread the damage across the entire party evenly, allowed them to survive with no party deaths.
So I'd say so far, 5e is built to allow for low level creatures to remain a threat to higher level PCs, provided you increase the quantity of lower level creatures attacking them.