There is one place where we can examine the performance of D&D books specifically, which is Amazon. And what we see from Amazon is spectacular sales, with the PHB regularly breaking into the top 100 and other D&D books also performing very well.
Obviously Hasbro is pursuing a multi-platform approach to 5E, with e-tools and video games and cross-promotions and the whole shebang. I have no doubt they've got money flooding in from many channels. But there is no reason to believe they are doing poorly with plain old book sales, quite the contrary.
Like I said: Ryan Dancey's strategy was a good one in 1999. Maybe there were better ones, but his was good enough to launch 3E to considerable success. But that doesn't mean it's a good strategy, let alone the best strategy, in 2020.