I guess my perspective on things is this...
In 2008, Danny Ainge took a gamble. He had a Celtics team that wasn't going far, with a star player who had a few years of prime play left. He had just drafted a very talented big who seemed to have great upside (Jefferson) and had some other nice pieces too. He probably could have tried to develop some of his young guys, and maybe the Celtics would have been a first round exit playoff team for the next 4 or 5 years.
But he took a gamble. Traded away half hours team of role players and prospects for two 30 years old all stars. From day one, the team really only had a 3 or 4 year window, given Ray's age. It was a pretty big risk. If it doesn't work or then Boston could be bad for a long time, for nothing.
But it does work out. The Celtics with banner 18, made a couple of finals appearances, 3 or 4 deep playoffs runs. It was an amazing time to be a Celtics fan, it really was.
Then after it he traded in those stars while they still had some value, and got back some assets that helped establish a future. The Celtics studied bad for a couple of years, then head a couple of years of first round exits - not optimal.
But, if I had a chance to take back the kg/ray trades in order to eliminate those couple of Lottery seasons we ensured, wound I do it? No way. Banner 17 is worth more to me then two or three years of stinking lottery ball.
So, if I had to sacrifice the future a little for a chance to have two or three more shots at a title, then by golly I'd do it.
Melo puts us there. As we are now, we are not a team Cleveland wants to face in the playoffs. They're better then us and they know that, but they also know any series against us is going to be hard fought, emotional, and a battle every step of the way. We add Melo and suddenly we become 'that team' that they know full well it's capable of beating then in a 7 game series. If we then add Vucevic as well, suddenly our roster is arguable better then theirs is. Now, we make the Cavs sweat a little.
And we become a far more attractive player destination too. GS and Cleveland wouldn't pull the personalities they do if they weren't so stacked. Veteran players will start wanting to come to Boston on cheap contacts for a shot at a title, etc. Our fortunes go up.