I agree that M. Gasol is looking better and better in Memphis- he has clearly worked on his game and is developing into a solid player. I also know P. Gasol hasn't played yet, but that will come very soon and he will be just fine.
I guess the main point of the trade in the first place was that it was so lopsided at the time. M. Gasol was not considered even a decent prospect, so to get P. Gasol for him was pretty much a joke. At some point in time, maybe they will be more comparable (in terms of talent), but if anyone ever thought that would be the case, he would have gone much higher in the draft.
I guess the best scenario would be the unthinkable- P. Gasol quickly declines, while M. Gasol becomes a star. Even in that scenario, though, it still doesn't take away from the principle of the original trade...and you are right, Bynum would have been a more fair trade at the time...