IIRC the sales numbers Ben Riggs showed from the TSR era shows that there was a brief "fad" period of D&D sales during the early part of the Basic/AD&D lines. Once the fad ended, sales began a slow and steady decline. When combined with the massive release schedule of AD&D 2e, this helps explain why TSR went bust (although there are other reasons, of course).
Moving on, I'm led to understand (if memory serves) that in a recent seminar where he discusses the fate of 4e, Riggs describes the slow and steady decline in aggregate sales of D&D as actually continuing across the WotC era up until 5e came out: 3e/3.5 sold a bit less than AD&D 2e, and 4e sold a bit less than 3e/3.X. (Not enough, I should note, to meet any reasonable standard of "failure", preposterous absurdities still espoused by edition warriors notwithstanding.) Of course, the release schedule of both 3.X and 4e was quite a bit less aggressive, so each individual product did better in terms of sales.
I'm inclined to say that 5e has come the closest to matching the kind of D&D that gamers actually want to play, which is why it has sold so well and for a whole decade, to boot (as opposed to the Basic/AD&D "fad") - Basic and AD&D were too restrictive, and 3.X/4e were too fiddly. 5e has hit a sweet spot. I daresay the developers don't want to mess with that too much.
So it's IMO not accurate to say that the release strategy is D&D withering on the vine. At the same time, the game always seems to me to be a bit "underdeveloped", as it were - (edit to add) as if it hasn't had enough time for really rigorous playtesting and development - that is to say, not enough money is being spent on developing the game (end edit). A more aggressive publication schedule would result in content being even more "half-baked" than it already is.
I would think that Colville isn't quite right, but he isn't exactly wrong, either.
Also edit to add: Apropos specifically of adventures, I believe Perkins or Crawford stated that those are timed for release such that a table playing circa four hours of D&D a week that stays reasonably "on task" to some extent can play through an entire adventure and then start the next one more or less right when it comes out. To my mind that's a very sensible publication schedule!
I am of the camp that the release cadence could still use improvement, but we dont want 2nd, 3rd, or even 4th ed levels of product bloat.
