The problem is that Boston has swap rights on their 2017 first round pick and Boston outright owns their 2018 first round pick. So blowing it up is pretty unappetizing for them, since they won't reap any draft benefits from it for the next two seasons.
Honestly, this might be the worst trade in NBA history:
Boston gives up: Paul Pierce, Kevin Garnett, Jason Terry, D.J. White, 2017 conditional second round pick
New Jersey gives up: Gerald Wallace, Kris Humphries, MarShon Brooks, Kris Joseph, Keith Bogans, 2014 first round pick, 2016 first round pick, Celtics' option to swap 2017 first round picks with Nets, 2018 first round pick
If Boston builds a juggernaut, the Nets will have a huge hand in it.