Brandan Wright has had 4 very good years for Dallas, playing around 18 minutes a game off the bench. This year, at age 27, he is having the best year of any NBA bench player. His PER is 26, WS/48 among the 10 best in the league. He has been awesome.
So, as a big forward, your best years are going to be ages 27-31. Can Brandan Wright be a starter? The answer is "probably not." I can think of no example where a guy at age 27 went from being a good bench player (true bench player, not some guy playing 30 mpg off the bench) to being a good starter. None. I am sure it has happened somewhere, but I don't remember it.
So chances are that the Celtics just picked up a guy who will give them 20 really good bench minutes and nothing else. There is value to that. The real question is whether the guys who take over Rondo's minutes can approximate Rondo's production.
But anyone thinking that Wright will be a 30-35 mpg starter should think again.....or at least provide me some historical basis for your position.