B-4 was no more "empty-headed" than a developmentally disabled human. You wouldn't say such a human's personality didn't exist or didn't deserve to exist, or that it would be okay to overwrite it with a smarter person's brain.
Well, unless you're Julian Bashir's parents...
Bashir was just made smarter, Uhura was just relearning everything erased from her brain, and Spock was just getting his soul/memories back.
B4 is not Data, he does not have the same body as Data this was shown by him and Data interacting as separate people so no Data shouldn't just be made to steal his body because some fanboys don't want to deal with the reality of death in Star Trek.