Ok, a couple of points to keep this in perspective. The worst selling 5e book is still selling considerably better than the best selling Pathfinder book. EVERYTHING that WotC has published for 5e is selling better than every other company's books. It's not a case of a couple of books doing well, it's a case of EVERY book doing better than anyone else, full stop.
Let's not forget that while the PHB is selling in unbelievable numbers, the DMG and the Monster Manual aren't actually all that far behind. As I write this, the DMG is sitting at 244 in all books and the Monster Manual is 277.
Think about that for a second. Three years in, the "DM books", those books that have never really sold all that well previously because, as the common wisdom held, there are a lot more players than DMs, are actually selling fantastic numbers. That means that the number of DM out there has to be skyrocketing. We're on a NINTH printing for the Monster Manual and a fifth printing for the DMG.
Why on earth would they even consider a new edition? The reason we got 3.5 and then 4e so quickly is because sales had died. By this point in 3e, 3.5 and in 4e, the editions were done. Three years into each of those editions, they were either announcing or had already released a new edition. To the tune of millions and millions of dollars sunk into each new edition.
Good grief, they're licensed to print money right now. This is probably the most solvent, most successful D&D has ever been.