Good piece!
In terms of comprehending B/X's significance, I think we might pause a little longer to appreciate the massive sales numbers Ben Riggs documented, and thus the ubiquity of the edition in the fad period (through 1983) and the years immediately thereafter.
While the popular sentiment for most of us was indeed to "graduate" to the "more grown-up" AD&D, between OWNING and having read B/X, and the incomprehensibility of many of AD&D's sub-systems, most notoriously initiative, a huge percentage of AD&D players, especially younger ones, were in practice kludging it with B/X. A fact many of them realized a couple of decades later when they came back to the older editions during the OSR movement. I think this has been a big element in the popularization of B/X within said movement. While AD&D was massively prominent in the early OSR, a huge share of the folks coming back re-examined AD&D and B/X and realized that the way they played back in the day was actually much more akin to B/X, just using expanded AD&D content.
On a related note, another reason for B/X's renewed popularity is simply Moldvay's tasteful and competent editing. Unlike AD&D, for example, B/X tells you right in the rules how long paralysis lasts, and gives PCs a way to fix it (by granting that capacity to Cure Light Wounds). Between this and other common situations like initiative, many OSR fans decided that B/X offered a better rules framework for their needs than AD&D.
And in the spirit of proper grognardism, a few quibbles with your post!
1. Let's also credit and acknowledge Steve Marsh, who co-wrote the Expert set with Zeb Cook.
2. Terminology-wise, the official (and IME dominant) nomenclature for the B/X and BECMI lines was "Dungeons & Dragons", as opposed to "Advanced Dungeons & Dragons" (the REAL game, for grown-ups). Not "Basic D&D", which would have been perhaps slightly clearer, if a misnomer for the Expert and later sets.
3. I'd argue that you've gotten a little bit of the
chronology backwards. The OSR preceded and birthed retroclones. The OSR sprung up a year or two after 3E was released, first on message forums like Dragonsfoot (which started in 1999 in the wake of more fan-friendly WotC internet policies and quickly ballooned in popularity, with Gary Gygax participating in extensive Q&A threads, as he later did here on ENWorld), Knights & Knaves Alehouse, and Original D&D Discussion where entrenched never-left-AD&D grognards welcomed folks who had returned to D&D with 3E then decided it was too rules-heavy, and/or didn't scratch their nostalgic itch properly. The first uses of the acronym can be traced back to 2004 (coined by Trent Foster, IIRC), but the movement was already building for a couple of years by then. This movement, combined with the original books not being available for legal sale, created the
demand which birthed the retroclones.
4. As a point of clarity, Labyrinth Lord is not a direct clone of B/X, but rather a mix of B/X and AD&D, immediately noticeable in the expanded armor and weapon options, Clerics getting a spell at 1st level, the level chart going to 20 and the spells to 9th. The main elements it keeps from B/X are race-as-class and simpler systems than AD&D.
5. The Moldvay Thief does suck, and is probably his biggest editorial failing. It's actually identical to the Menzter Thief in the first two sets of Mentzer, then the Companion set retroactively bones the Thief even more by errata'ing the advancement tables and re-jiggering the skill numbers. An error carried forward into the Rules Cyclopedia. A better choice, of course, would have been to give the Thief new capabilities at higher levels, as was teased in the Cook/Marsh Expert set.