Too many posts to read them all, but I did note someone saying why didn't Picard have Data take the new body.
Data wanted this to happen. He got to be a part of the human condition, and being mortal IS a part of it.
"Time's Arrow"... in Ten-Forward, Data and Geordi are talking, and Geordi is bothered by the fact Data's fated to die.
"This has got to bother you a little." Geordi
"On the contrary, I find it comforting." Data
He goes on to say he could have theoretically lived for centuries, maybe millennia. But all it did was just reinforce the fact that he's artificial, and now that he could in fact die, he is one step closer to undsrstanding humanity.
Data obviously survived that, but the point is right there. This finale rectified the mistake of NEMESIS. Not in bringing him back from the dead... which I would have been THRILLED to see happen... but in giving his death MEANING. Picard got to say goodbye to him, heal, and move forward, while at the same time Data getting to say a proper goodbye to him. If I can't have my scifi hero back, then I damn well want him to have a meaningful, beautiful farewell... something he more than earned and deserves. And we got that!
(Hell, I'm starting to tear up again thinking about the scene.)
I wish I could thank Michael Chabon in person for giving us that gift.
Side note: I don't know if anyone has thought of this, but maybe another reason Data wanted to be gone was because being stuck in that limbo for so long might be torture? If a fraction of a second can seem like an eternity for an android, what must YEARS feel like? One could also make the argument Picard was giving him a merciful death. I prefer to think, and I still believe, that everything I stated before and what we saw on screen is the real reason.