I'm not surprised - I always argued at the time that Perkins was going to leave anyway, since it was pretty well documented that Boston wouldn't be able to pay him what other teams would potentially offer.
When I made that argument at the time the response from people was generally something like "I don't care if he was going to leave, keeping him to the end of the year could have won Boston an extra championship".
But you have to put yourself in Danny's position.
He had Shaq and Jermaine O'neal - both were at the end of their careers and were riddled with injures. Perkins was sidelined with injury, and nobody truly knew when he'd come back or how effective he would have been when he did return. If Danny kept things as they were he would have been putting all his faith in two broken down vets (Jermaine and Shaq) and an injured big who may or may not return as anything close to what he was. That's a gigantic risk. And then on top of that, you also risk losing Perk in free agency and getting nothing in return.
On the contrary, by pulling the trade he got back an extra young talent who appeared to have significant upside (Jeff Green) and also got back a healthy starting caliber big (Kristic) who gave Boston much needed security at the center spot. Both of those guys were also quite young at the time, so it also made sense for that reason - as Boston were getting pretty old, and it seemed like a couple of sets of young legs might help add some life in to the team. When you put yourself in his shoes, at that very moment in time, the decision was pretty much a no brainier.
Now as things turned out, the logic of the deal was made all the more clear when both Shaq and Jermain struggled with injuries and missed bunches of games - unfortunately unforeseeable bad luck struck and Kristic also ended up getting injured. There is no way Danny could have foreseen this, but sometimes bad luck just happens, and there's not much you can do about it. If Kristic stayed healthy maybe Boston would have won the title that year, and then the Perk trade would have seemed like a great move.
It's easy to criticise Danny when you've seen the future and know how things are going to pan out. But he doesn't have the luxury of time travel - he has to make the best decision he can at a specific moment in time, armed with the best knowledge he has available. I always felt that he did the right thing.