Honestly, I think the rules as written can work fine. Things get strained if rests are too easy, and if magic items are handed out too freely, but even then a clever DM can make things very challenging. The problem is most DMs don't play monsters in an intelligent way, even the ones that should be intelligent.
Though as a fun exercise, I'm going to make a sample adventuring day for a group of four level 15 characters, assuming vanilla rules. Seven fights, one hard and six medium, two short rests. Note that this is just for fun, and the fights probably won't make sense, but I think in about ten minutes I can scrounge up a challenging day for a group. Obviously for a dungeon more work would need to be put into making the encounters more rational given the environment.
Encounter 1: Twenty-five giant poisonous snakes, six giant octopi, and an otyugh. The snakes slither up through holes in the ground.
Encounter 2: Four ghouls, a giant ape, and a mind flayer.
Short rest
Encounter 3: Two hobgoblin devastators in the back, two quicklings, and six ogres as a front line blocking the way.
Encounter 4: Two earth elementals, two fire elementals.
Short rest
Encounter 5: Four flameskulls and six firesnakes, the flameskulls open with readied fireballs when the PCs walk in. Flameskulls are always ready!
Encounter 6: Eight winter wolves.
Encounter 7 (Hard): Three ogres, three hobgoblin devastators, and a tyrannosaurus.
Long rest
The above was picked partially at random, and partially from my favorite things from the Monster Manual. I think those encounters could very easily challenge a group of level 15s.
