Eben has spoke of the hardware limits of the platform and the foundation has been putting a lot of effort into the software over the past year. Up until recently the software was not taking full advantage of the hardware.
The software has gotten a lot closer now, and work continues on optimizing and polishing it. With just a bit more work on that the Pi will meet the intended goals originally envisioned for it quite nicely. It's even fair to say it will exceed them.
A Pi3 actually does function quite nicely as a desktop PC right now. It runs Libre Office, plays YouTube videos, and the Chrome web browser filled most of the gaps that stood out in their Epiphany browser.
What you can do with it beyond that is pretty amazing so it really doesn't make a lot of sense for the foundation to be spending a lot of cash and effort to produce a more powerful hardware platform until something in that domain changes that makes that easy and cost effective.
Also, the foundation has been working on making their software (PIXEL) run on other hardware platforms (https://www.raspberrypi.org/blog/pixel-pc-mac/) so they are providing a hardware upgrade path with that route.
Finally, there are 3rd party hardware boards that are more powerful available that will run PIXEL now (http://www.banana-pi.org) and this might give them a boost in sales which would be a good thing.