The defense will need a complete overhaul soon as you state. Might as well try and go for it now when you can still get something out of the older guys. The relative youth on offense though is encouraging - Bushrod, Bennett, Forte, Jeffrey, and heck Marshall/Cutler only 28 & 29 respectively.
Emery banking on Trestman extracting the value out of these guys as he knows that's where most of his younger talent currently lies. A huge if though.