Parmandur
Book-Friend, he/him
When your game is in decline, then anything to boost the businsss around it is a sound business decision.
That's not a PHB2 either. I understand that subclasses are the goal NOW and for the forseeable future. But for us to get to a true 6e then 5e has to be in a significantly declined state. If 5e is in that state then they will attempt strategies to save 5e first. One strategy at that time is likely to involve more classes to drum up excitement from long time 5e fans. Bored fans tend to not buy new books of any kind afterall. I think it's only after they have pulled out all the stops of trying to save 5e that we will see a 6e.
Why would they put out a book that is proven not to drum up business (indeed, demonstrated to depress business) for the purpose of drumming up business? That's a contradiction of terms, and no business conditions will change the logical terms to make that circle square.
I doubt there will be that level of boredom any time soon, and at any rate they will do what other properties (like Monopoly) do when trying to receive interest and go for something like Star Wars D&D or Zelda D&D, rather than something illogical like a PHB2.