Thaumaturge
Wandering. Not lost. (He/they)
As for the PHB, DMG, and MM, my guess is that they will revise and reprint them every few years to reflect rules additions, tweaks, etc. Or maybe only the PHB. My guess is that it won't be "5.5" but more like "5.2" in 2-3 years, "5.4" in another 2-3 years, etc.
This. I got the same feeling after reading the coverage yesterday.
So rather than saying they're "planning 5.5 already," my sense is that they are trying to make room for inevitable rules tweaks and adjustments, which will always be reflected in the free Basic D&D document. I mean, we want them to continue to improve the game, right? I think they may have stumbled upon the best way to do that without pissing people off with a major revision every few years - they're offering a high quality PDF for free that will always be up-to-date.
Not only will the changes to the core system be in Basic, but any changes will be well communicated through public playtests of those changes, after the community has requested the changes in the first place.
Things are so much better than "planning 5.5". They know iterations on 5e will, eventually, be a necessity. They're implementing systems so the community determines the timing and content.
That's my, optimistic because we're in a thread about being excited(!), view.
Thaumaturge.