This wouldn't bother me much, except once you hit level 7+, your choice in "level equivalent" foes becomes thin; you're almost always forced to use lower level foes on your opponents after a while. It'd be nice if we got some decent high-level monsters; but so far all the new monsters from modules (save the unique ones) are all > 6. It gets really hard to throw armies at the PCs encounter after encounter...
Four CR 5 foes (Elite Drow Warriors, elementals, etc.) remain Hard or Deadly for four PCs right up until level 13. By twentieth level it takes eight of them to make it Hard. Being outnumbered 2:1 hardly constitutes fighting an "army," and there are tons of CR 5ish foes in the MM.
On the other hand, I actually
like armies in 5E. Reminds me of the good old Gold Box games.
Bounded accuracy also presents a problem. With my group, there is one character that had full plate by level 3, and has been riding on an AC 23 or 25 (if using a buff) ever since. What this means is that I could never really challenge that player or the group with "several weaker monsters" compared to "fewer stronger monsters" because the weaker stuff all had +3 to +4 to hit, which means the paladin could wade into combat with impunity.
Normally paladins max out at AC 21 (23 with Shield of Faith) so I assume you must have given him a magic shield or something, yes?
If you want to challenge someone with high AC, you can set up the terrain such that the character has to be concerned about enemies Shoving him into quicksand, off cliffs, etc. (After all, even The Dread Pirate Roberts got shoved off a cliff by a girl. "Aaaaaaas yooooouuuuuuu wiiiiiiiiiiiiish!" Inigo couldn't touch his AC but Buttercup got lucky on her Strength (Athletics) roll.) Pushing an enemy into a terrain occupied by some dangerous beast is another variation: maybe the Otyugh doesn't come into play unless someone gets pushed down the slope into the peat bog.
Also, bear traps, caltrops, covered pits, flaming oil, etc., still remain effective no matter what AC is. Not to mention object manipulations like cutting the bridge when someone is halfway across, or dropping a portcullis that isolates one PC from the rest of the party.
Goblins playing hit-and-run (advantage on most attacks due to Nimble Escape) will annoy a PC even if they hit only on a natural 20. Ten goblins means about one crit per turn, and by level 7 the PCs are "expected" to handle about twenty goblins at a time.
The DMG has a Disarm variant rule which I quite like. Even if they can't hit your AC, being disarmed is quite annoying for most pure warriors. The paladin will have to Smite things with his bare hands.
If the other PCs are around, the goblin conga line can just snake past the paladin and hit the squishies. If they are actual goblins wish actual bows, you don't even need to physically move past them, just unload your arrows into the wizard and bard instead. The high-AC character then needs to either respond to that threat, or tell the rest of the party to go ahead while he deals with the threat, in which case the above tactics all apply twice as hard because he now has no support.
I'd argue that you're obligated to include some of this kind of material in your campaign regardless of whether or not someone actually has a stellar AC, so that you're shaping the game world instead of metagaming against a specific PC. Not every adventure has to take place on a windy mountain trail, but consider making goblins prefer windy mountain trails and slimy peat bogs in which to build their warrens because they are so defensible.
