Right, I agree that GSP is better suited to WW frame wise (I'm sure he could eventually make LW, but he would be very "sucked out" and likely lose some of his explosiveness). But, what's he gonna do? Win the MW title and then immediately vacate it? What's the point in doing that?
Should he win I see him staying at MW until he either gets beaten, or at least has a few title defenses (Dana and the UFC may try to protect him by giving the smaller less powerful contenders a shot instead of the big boys in the division).
I don't disagree with being a little shocked that he is coming out of retirement for this fight. I too would like to remember him for his amazing legacy at WW rather than to see him potentially tarnish that and his image by possibly getting mauled by fighters in a bigger weight class.
That said, Randy Couture came out of retirement (having lost to Liddell twice at LHW) to fight Tim Sylvia at HW (who at the time had essentially cleaned out the division) and won. So, such an upset has occurred in the past. Yes, it should be noted that Couture had fought at HW prior to that fight, but still, the example still holds true (since he had retired a LHW).
Like I said, I think of all the MW top contenders Bisping is the one GSP has the best chances of beating. Neither fighter are known for their power, Bisping does not have a huge reach advantage, and as far as comparing GSP to Sonnen they are nothing alike in terms of their overall fighting skills or takedown set-ups.
Sonnen is a flat out pathetic striker, and his submission skills are sub par as well. He basically just dives in for takedowns and only is successful with them because he is relentless, willing to take punishment to get them, and is a good chain wrestler.
GSP on the other hand is (or at least was) masterful at blending his striking and takedown skills (arguably the best ever); always keeping his opponents guessing and utilizing superior timing to get the takedown rather than just pure brute stubbornness.
When you know your opponent has only one threat of attack (takedowns) it is a lot easier to stop them than if you have to constantly worry about multiple potential avenues of attack.
And right you are about Sonnen not being able to hold Bisping down, but again, Sonnen is pretty much just a wrestler who chose to compete in MMA. GSP is a well rounded Mixed Martial Artist and even if he can't hold Bisping down he may still wind up with The Count's back and choke him out, or use Bisping's focus on getting up to land some solid strikes.
I think the interesting thing in this fight will be to see if Bisping can keep GSP off balance and take him down (and if he can what he can do once the fight hits the ground).
Finally, I honestly think GSP had lost his passion for fighting by time he fought Hendricks and so his heart wasn't really on that fight. Hendricks also was stylistically a much more dangerous opponent for GSP than Bisping (crazy power in both hands, highly decorated wrestler, tough as nails, extremely aggressive, and a fire plug of a build) IMO and styles make fights.
We'll have to wait and see how it plays out come fight night though I guess.