Inchoroi
Adventurer
Right up front, I want to say that this could very well be a generational problem, and not nearly universal to most tables. Secondly, I think the published adventures (Tiamat, CoS, OotA, SKT) have a lot of good things about them, and for the most part we've enjoyed playing them.
However, here is my problem. Probably the biggest problem. All of those published adventures go to level 10 or 15. When you're done with them, your PCs are pretty much done. Time for retirement. I played TSR D&D as my main edition from 1981 to 2012. One thing about TSR D&D is that the level ranges 4-10 took a long time. You could go on several adventure modules and still be within that level range. Just finished the entire Slaver Series? No problem, we can still go play Ghost Tower of Inverness, and when done with that, go play White Plume Mountain. The DM would tie all of those together in his or her own campaign world with a central plot. Maybe after completing the Slavers, your PCs enjoyed their spoils while you went on another campaign with different PCs and a different DM, then came back later to reuse those original PCs in another adventure.
I really miss that with 5e's official published campaigns. The campaigns above that I've finished? I really doubt I'll ever use those PCs again. We've never really played end game levels anyway, not that there are any epic level adventures put out yet even if we wanted to... And you can't really take a 15th level PC and play them in lower level adventures.
With TSR D&D, we'd play a PC for an adventure, then a different PC in a different adventure. It allowed us to get more hands on experience with different character themes, and choose our favorites without spending dozens of sessions on the same campaign with the same PC. Maybe after playing 4 PCs in 4 different modules, I decided to use Norlay as my favorite when we did future adventures. In the current campaigns by WoTC, you can't really do that. It would be like switching PCs every chapter, and that doesn't really jive well with how the adventure progresses organically.
Maybe that's why I would really like to see a return to the old school style modules.
You know, I have to agree that it is a slightly annoying problem. To suggest something different, though, for you, the OP: take a look at Frog God Games current offerings, and Necromancer Game's past offerings. They have a very old-school feel. You'd still have to address the leveling speed, but at least you'd have the right adventures for what you want.
I've considered changing the requisite amount of XP for each level, but suspect that my players would kill me if I tried.