Every reason to want to regret the trade only manifested itself after the trade was made. And even now, why would anyone want to undo the trade and return to the status quo of the last decade? To lament the trade for Bynum is to lament a desire to win a championship. Sometimes, it is necessary to take on risks. And while Bynum was a bit of a risk, the odds of him never playing a game in a Sixers uniform were slim (those odds may be increased a bit now, but again, hindsight should not be the basis of judgment).
With Bynum, the Sixers have a direction. With Bynum, the Sixers have potential. With Bynum, the Sixers can be championship contenders.
Therefore, when push comes to shove, even if Bynum never plays a game this year, signing him to a max contract is still a smarter move than letting him walk at the end of the season. Superstars don't just grow on trees. Letting Bynum walk and saying, "oh we'll just find another superstar big man in the 2013 off-season" is easier said than done, even more so when you eliminate Dwight Howard and Andrew Bynum from contention.*
