For a couple of years following 4E's release, the D&D brand slipped so much that another game (Pathfinder) became the #1 RPG on the market. It was the first and only time that has ever happened.
If I recall, Pathfinder overtook D&D once D&D after it was announced it would end and stopped being printed in prep for the next edition of 5e.
There normally is a drastic decline anytime a new edition is announced, and stopping the outpouring of new material almost always is a good way for sales to decline.
That's a situation which makes it easier for another brand to outsell it for awhile...especially if materials are being printed for it while none are being produced for D&D. The fact that D&D didn't sink further is an attestation of how big D&D is on the market.
This doesn't mean that sales were lower than in earlier editions (my guess is that while 3.X had around 5 million, 4e had more like 2-3 million players, but that's a guess).
What I think was heard is that Hasbro set expectations for core brands to be selling at a minimum of 50 million dollars a year. Some thought core brands should be at 100 million a year. D&D wasn't making that much and Hasbro was a little disappointed in that aspect at the time. This probably caused some difficulties and reorganization of expectations and how the brand was presented.
I imagine the goal is still to somehow get the D&D brand (that's brand, not game) to attain a 50 - 100 million mark by various means and retain that each year. Whether they are suceeding or not is probably only known by those in charge of that, but it would no longer have been expected to meet it immediately upon 5e's release (or so I would think).
MTG is still the big money maker, and hence the bigger focus these days in any case. D&D has a large interest in it's following, but I imagine MtG is the one which has a bigger interest from Hasbro these days.