The most recently-published adventures are so far superior to the early ones there is little or no comparison
This IMO is quite true, and very telling. I think the Delve format for the presentation of encounters had quite a lot to do with that. However it was almost as if the writers forgot that there is more to D&D than combat, and you can't blame the edition, or the long format for that. The latter adventures bear this out, and if you look at adventures in Dungeon Magazine you also find some gems that don't forget it.
The monster stats were not the only reason the early modules are not good, however: for some reason the writers of the early modules (oh God P3 Assault on Nightwyrm Fortress I mean you!) complete forgot what a roleplaying game was supposed to be and just wrote out a very long list of dubiously-related combat encounters.
Very true, and I really think that the Delve format is also the significant cause of this. When your format for every encounter is two pages long you are basically designing set-piece combats for everything, and that eats up a lot of space that could be used for expanding NPCs, background events, etc. (the things that make roleplaying games more than just combat simulators). The game does not have to play like that. It doesn't play like that at my table, and I've seen a lot of DMs here demonstrate how it does not work like that with them either. It is also obvious when you see later adventures like Gardmore Abbey which definitely did it better.
In another thread Cordell and Schwalb both mention that 4e does set-piece combats well, but that encounters with insignificant creatures (4 kobolds) don't play as well. I call shenanigans on that. I have seen plenty of DMs run "insignificant" encounters as they are meant to be run, as "insignificant". What the hell did these designers think minions, and lower level creatures are supposed to be used for?
You use the tools that the game provides to get the results you desire. You tweak the encounter to fit your desired needs within the adventure. Not every encounter needs to be a set-piece, level appropriate, or EL+(2 to 4) encounter. So why do you design an adventure where every encounter is just like that? If they are blaming the rules for inflexibility I really have no sympathy for them. The rules present a very robust framework that is very tweakable to achieve desired results. Now, if WotC editors and their policies forced that unworkable situation then we are talking about something different. You can't blame the game engine for not performing the way you want it, if you make policies that prevent it from doing so.
Cordell was the one who, along with Mearls, wrote Keep on the Shadowfell. He also wrote Assault on Nightwyrm Fortress. Both of those adventures left
a lot to be desired. However, this decline in making adventures interesting did not start with 4e. Cordell also wrote Bastion of Broken Souls and Heart of Nightfang Spire - IMO, two of the worse adventures I saw during the 3e era. He also wrote Sunless Citadel, a good adventure by most reviewers estimation, and I agree.
So was it the game engine, the adventure writer, or the publishing rules from WotC that created the problem?