I agree that they need to rebuild. But that is easier said than done. Danny can't just say "here are my old guys, give me your young guys".
Rebuilding in a process, and a delicate one at that. You need to be smart with every move. Every move needs to be examined from every angle, to determine the impact of players they are giving up, impact of players they are bringing back, and the financial and cap implications as well.
Right now, I think Danny's biggest goal should be to try to rebuild some assets. Currently they have very few valuable assets for rebuilding, and he needs to get creative to change that, while not putting their future cap situation at risk (especially given the uncertainty of the new CBA).
He needs to keep his eyes open for opportunities to grab a young star, but I think his more likely approach will be to try to keep the team competing for the next two years, without taking on any long-term contracts, and trying to gain some extra assets (young guys on rookie deals, or draft picks). Then trying to really rebuild as Pierce and Garnett come off the books, and he has a better idea of what the next CBA is going to look like.