I've been DMing for 35 years. I would like to consider myself experienced. This is not a simple "re-calibrate on the fly". I'd have to rewrite everything in order to make something like HotDQ/RoT playable for a narrowed level range, and that's a LOT of WORK. For one, it doesn't make any sense to be reaching the culmination of that campaign when you're still only level 7ish or so, which is the pace I'm happiest with when factoring in the amount of time actually playing vs. how often you level up. Fighting dragon lords, dragons, and possibly Tiamat herself at level 7? That's a lot more than just modifying on the fly.
No, what I'd have to do is what I'm doing with SKT. I.e., I've inserted B5 Horror on the Hill to replace the dripping caves because I feel the level advancement in chapter 1 is WAY too fast. I'd have to do that with every chapter to get the slowed level of advancement I'm talking about here. At that point, you're really not playing the published official campaign anymore, but this giant rewrite.
Everything you say is true.
Perhaps the most egregious case is Legacy of the Crystal shard, which officially is a, get this, level 1-3 adventure
"Luckily" the D&D Next stats are very wobbly, so they need replacing anyway. And retooling a level 3 encounter as a (more more appropriate) level 7 encounter isn't that hard anyway.
But I digress. High level play is something completely different. You simply can't just change the numbers but otherwise run a level 5 encounter at level 15.
Or vice versa, as in your case.
All of this in addition to the basic fact that by this time you're pretty much doing all the work yourself, which purchasing an official module was supposed to prevent.
I'm running the end of Out of the Abyss, and there is precious little that can be run as-is there, I can tell you that.
So I guess we will simply have to wait for Wotc to get this "one big shared adventure" phase out of their system.
Although I admit it doesn't look good, since the thing about the "shared" part isn't so much the community building of everybody playing and discussing the same adventure...
...it's the everybody
buying the same publication part of it they're gunning for.