Good sales aren't reason enough to keep making it, they might think making it a crossover will make it more competitive to the Pathfinder and Explorer which went unibody years ago... along with the bigger GM crossovers that replaced the minivans. The 4runner is pretty close in size and function to the Highlander, so I wouldn't be surprised if Toyota axed it altogether.
I never know what Toyota is thinking, because they made a dumbass statement about a year ago about having a commitment to BOF SUVs, then promptly killed the FJ Cruiser not long afterwards. I sometimes think that their US marketing department and the mothership don't speak to each other.
The cynical viewpoint is that there will always be some kind of Tacoma for those who want to go off road, and the 4-door versions can be appointed as nice as the 4Runner--so for the small percentage of people who use these as overlaners, 4x4s, rock crawlers, etc. there will still be a Toyota... while the rest of the mall crawler crowd will be better served with the fuel economy and car-like ride and dynamics of a crossover.
I know that the next vehicle I will buy (second truck) will very likely be a Tacoma. Still haven't decided if I want the current gen or the next gen model but lack of finances will probably steer me towards the current one.
Just remember that as a segment of the car buying public, Toyota really doesn't give a damn about those people who really care about the 4Runner being BOF because we're a tiny percentage of the buying market... and most 4Runner customers would probably love a crossover variant. I'm willing to bet many can't even tell you which are the drive wheels or what BOF means.
/2 cents.
**** you Toyota.