True. And I'm saying that mathematically speaking, that's probably a design flaw, depending on your goal.
Number inflation was a problem throughout 3e and 4e, with the designers not really cognizant of how fragile a d20 truly is as they went about giving various bonuses, often based on level divided by a small # (ex: 2, 3, 5) of varying types. And that applies just as much to your base skill difference.
Especially if you can then get, say, an ability score (let's say one is 6, the other is 20), an item bonus, a theme bonus, feat bonus, a divine bonus, or whatever other type of bonus you want.
So it depends a lot on what you want out of the system, but keeping the d20 in mind from the beginning (and thusly not doing a 1/2 level spread unless it's very much on purpose) is a good thing.
Well, the 3e system guaranteed you would have huge separations. The 4e system doesn't inherently. 1/2 level actually has nothing to do with it. It is stat boosts and willy-nilly stacking bonuses that IIRC the 4e designers claimed would not be larded into the game way back when, and then were.
So you could use the same system as 4e exactly, drop stat boosts, and allow nothing more than say 4 points total worth of bonuses that don't stack beyond Skill Training. That means the MAX difference at any level is 6 (stat bonus) + 5 (training) +4 (other stuff) = 15. That's a pretty wide range, but not outside a d20. If you assume a level+4 DC is intended to be a hard challenge even for an optimized character then L+4 Hard is a DC25+L and at-level Easy would be say DC8 (quite doable for someone with a total -1 modifier, but still iffy for that guy). That isn't far off from matching the existing DC table for 4e, though the progression past level 1 would be a bit flatter and diverge less.
Notice too that basically it is not really different from the '5e system' as it is now apparently configured. A guy with say a +10 (which you'd expect as about your best bonus at level 1) doesn't need to make a check on an easy task, needs a 3 on a medium task, and a 9 on a hard task. Untrained 8 stat needs 9, 14, and 20, so he's not really going to bother with a hard task unless he MUST.
Now just throw in the concept of "you can lower the difficulty by one with a good plan" and you're there (and really there's no reason that doesn't work in 4e, though it is more designed around SCs).
Finally throw on skill trick/knack/power stuff that lets you do extra tricks or say a feat that you can take to get some 'specialist' training that knocks certain tasks down a difficulty notch and you basically have what 5e is proposing.
There's not really a big issue with 4e's skill system and scaling in other words, except all the crap 4e dropped in for bonuses. If they'd used "drop the difficulty one class for doing X specific thing" instead, well, it would have been more stable etc.