While I do think that putting out only 14-ish books in slightly-less-than-7 years is slow, I think there's a far better--and, more importantly, far more objective--measure of the minimal productivity D&D currently has.
That ol' reliable, the Jury Duty Problem.
They had one person be on it. One, singular, person out for jury duty for a few months. Because that one, singular, person was not working actively at the time, WotC was (apparently) completely unable to finish the vitally-important previous-edition-conversion-document, which would have been excellent to have early on as it would smooth the transition to the new edition. And then, when we did get that document...it was incredibly barebones. Only a few pages, highly simplistic approaches (e.g. "if you have a 4e character, multiply their level by 2/3 to get the 5e level"....as if any of us couldn't figure out that 20/30 = 2/3....)
If having ONE person out of commission puts critical projects on hold for literal months, when the final product of those projects ends up being very nearly trivial, that does not speak well of the team's productivity. It says that the rate at which they're working isn't so much a choice as it is a limit; they cannot produce more material, regardless of what they want to do.
(Of course, I'd also argue that the repeated circling around the Ranger, and specifically the Beastmaster, without ever fixing it indicates an issue with productivity as well. Despite being a well-known problem, one the designers have admitted, we go months, sometimes years between new official attempts at fixing it and they never seem to quite get the job done. Their ability to internally playtest clearly still has issues, and that implies either they're doing it badly, or they don't have enough people to do it, or both.)