A player stays with an NBA team for typically, what, three years? We have this conversation a lot. Most of the current Suns roster will have turned over by the time they become relevant again.
I don't consider it "mortgaging the future" unless you're giving up a core piece. So if the package was Warren, Bender, and a MIA pick, what's the core piece? Maybe Warren, and don't get me wrong, I like Warren, but I don't see him as a must-have going forward.
On the other hand, adding a legitimate center now would:
a) allow the rest of the roster to grow into their proper positions, instead of always moving up in small ball,
b) allow the coaching staff, if they are competent, to establish a culture of team defense, based on the presence of a real rim protector and defensive rebounder -- you know, the role that Chandler plays in the 25 games per season that he's healthy,
c) allow management to get an accurate look at the potential of the young players, and make better decisions about where the real gaps are in assembling a contender,
d) improve franchise morale by getting out of this era of endless suck, and becoming a credible destination for free agents again.
To me, that's a lot of upside if the only real cost is Warren.