the lives that would have been saved... I know it sounds cold, but I'm sure anyone on that DL flight would have been willing to give up their lives had it meant saving thousands on the ground.
There wouldn't have been thousands of lives saved. Only about 600 of the 2,800 dead at the WTC were occupants of the south tower. While many rescuers, probably 250 at least, died in the collapse of the south tower, most of them would have been in or near the north tower had UA 175 not reached its target and would have died when the north tower came down.
When you add in the people on DL 2315 (BDL-TPA, not BOS-TPA as earlier noted), plus any people unlucky to have been on the ground where it and UA 175 would have come down, you're probably talking about a death toll of maybe 2,300 instead of 2,800. Obviously, that would be better, but it's not a difference in the thousands.
As far as physical damage is concerned, while of course the south tower would still be standing had UA 175 been in a mid-air collision, it's not at all clear whether the building would ever be usable, as it shared a common foundation with the (destroyed) north tower. This issue is at the center of a lawsuit concerning insurance payouts.
"Let me think about it" = the coward's way of saying "no"