Quick hitters:
-We still haven't seen what Favors can do when asked to be relied upon consistently. The times that he's had to stand in for a starter, he has done exceptionally well.
-Big athletes that have good simple instincts and won't blooper their teams out of games aren't common. When it comes to the only thing even close to a defensive keystone, replacing their net contribution is not easy. At all.
-He's still young. And he hasn't played that many minutes (go back to point one).
-He's not strictly a center and neither is Kanter. Worrying about the issue of playing two centers in the stretch four era would be valid if neither big could guard out on the floor and neither big could do anything outside of the paint. The Grizzlies seem to be doing okay despite having at least one of these problems (and have it badly). It also wasn't long ago that Gasol and Bynum were a nightmare matchup.
-Swinging off the idea that post-players are a dying breed: why would Favors supposed inability to guard the post (which is exceedingly overplayed by OP) such a big deal? Favors is most suited to guard whoever's most likely to drag a big out on the floor anyway, and Kanter (or Al, or Memo RIP) should be just find guarding one of the 5 guys left in the NBA that you can worry about.
-I don't think Favors' value league-wide is very high. And what does he get you anyway?
I wish he was showing more stuff, too. But let's not get ahead of ourselves. I cannot emphasize the first point enough.