The grognards have had their say, the Golden Age of D&D is long gone.
I have three answers to that.
1: The nadir of D&D was the 2e era when they were pumping out five splatbooks a month (most of it appeared to be Extruded Fantasy Product) - but the creative energy and interest was all with White Wolf.
2: This might be the dark age of
D&D - but for RPGs it's closer to a golden age. Those who want to play hack and slash have WoW which scratches that itch. Those who want rich deep settings have all the old material. Those who want narrative storytelling games have more and better games than ever before - that side of the RPG hobby has grown by leaps and bounds in the past decade.
Genuine Golden Ages aren't where everyone's united. They are where there is a bubbling mass of creativity and everyone is getting what they want rather than being forced into a box that doesn't quite fit.
As for Mongoose's opinions on the RPG market, their main recent original product has been Stars Without Number - a d20 space based sandbox setting and system or in other words D&D IN SPAAAACCEEE! Now correct me if I'm wrong, but wasn't that the design goal of Traveller in
1977 (a licence Mongoose owns). In fact I'd call Traveller their main product line right now - a minor update to a 35 year old system. Their other lines include Paranoia (1984 or so), a recently updated Judge Dredd game which they've shifted from D20 rules to Runequest rules, a multiplayer expansion of a line of fighting fantasy gamebooks, a fantasy game using Runequest rules, and some D&D (or rather Pathfinder) compatable books. There is nothing in that line that appears to be other than well placed ... for the gaming market of the late 1980s.
Off the top of my hat, Paizo are doing well, so are Evil Hat, Margaret Weiss Productions, Cubicle 7 Games, Pelgrane Press, Kenser, and the kickstarters from Monte Cook, Vince Baker, and for the TBZ translation. (WotC would be doing rather better if they put out some product...) Mongoose have problems finding the pulse because they are looking at the gaming market as it was 25 years ago and trying to produce almost the same products that would have worked then. If you can't find the pulse and others demonstrably can, either look at what they are doing and you aren't or take up necromancy.
Edit: And the supplement Mongoose is talking about that utterly bombed from a leading company? Almost certainly Mezobarranan: City of Intrigue. Which pissed off 4e fans by being ... not 4e, and didn't have a market outside that. There's a chance it was the previous book; The Dungeon Survival Handbook which is far the most ill-thought out 4e supplement and was over half full of advertising for other D&D products. If both those bombed
they deserved to. The failure of either or both should stand as proof that you can't just put out anything, call it 4e, and have 4e fans buy it.