In previous editions, PC's got more hit points, better to-hit bonuses, better AC's (from gear) and more 'special' ie, magical abilities as they advanced in level.
In order to fight monsters with more hit points, better to-hit bonuses, better AC's, and more special ie, magical abilities.
It seems to me this has always been the case during the 20 or so years I've run D&D. It's not new.
In AD&D, it was entirely a matter of your (Mallus's) choice to ensure that all 10th-level characters were going around with +5 magic, much less that all the monsters they met were likewise equipped -- a mighty unusual circumstance in my experience! Of course, to get the full 4E effect would have required basically ignoring the monster ratings and combat matrices.
I suppose I could likewise ignore the guidelines and procedures in 4E and substitute the 1E rulebooks ... but then, when I wrote of how I played, I would not really be addressing 4E design features, would I?
I'm not sure how "getting more stuff as I gain levels" equates with every 10th level character having +5 equipment.
Getting more stuff as I gain levels does, OTOH, sound pretty much like every single D&D campaign I've ever played in.
Never minding all the rules in 1e that specifically assume you will gain access to magic items - things like needing +1 weapons to hit and the like. But, this conversation has been beaten to death way too many times for me to try again.
I completely agree that it is a good thing for adventures to feature a variety of challenge levels. This helps keep adventures more unpredictable and less likely to become flat and boring (this applies to all editions).
This has nothing to do with actual improvement or a lack thereof. Facing lower level threats isn't a real measure of improvement, its picking on the little kids. Dealing more efficiently with threats on par with your ability is a better measure of overall improvement. Think of it like a batting average. If Joe the slugger has a .265 major league average and works hard to try and improve, facing single A pitchers and bragging about his new .375 average doesn't carry any weight in a major league game.
This one, otoh, is much more interesting.
Yes and no on the lower level threats isn't a real measure of improvement. Let's not forget, if I use lower level threats, my xp budget lets me use a lot more of them. And, because the scaling isn't THAT far out of line, I can use a whole bunch of say, -3 opponents and still make a good enough challenge.
But, again, this isn't what 4e encounter design is about. The DMG is pretty specific, as well as the Monster Manual, in showing that every encounter should include opponents from both sides of the fence. In other words, Joe the Slugger should face both Major and minor league pitchers in every single game he plays.
If you look at the suggested groups in the Monster manual, you see exactly that. Other than the solo's (obviously) you have mixed groups that run a fairly broad level range. Because the scaling in 4e is linear, it doesn't hurt so much to drop 4 levels with an opponent. You just use more of them.

Conversely, it doesn't wind up being a binary alive/dead encounter if you go above the par either.
Like I said, I think you are absolutely right that the scaling would make for very boring encounters if you always used par monsters. Fortunately, the designers also agree with you and specifically tell us not to do that.