As for the thread premise, I've been pretty vocal about Danny's failure in this exact regard. He was so fixated on that one tree in the forest that he missed all of the other trees in the forest. But this has been Danny's m.o. for a very long time. He just really really likes what he has and just doesn't pull the trigger unless he is guaranteed of a sure thing. And some of what he passed up wouldn't have cost all that much. I mean the Paul George trade was something like Crowder, LAL/SAC pick (eventually Langford), LAC or MEM pick (eventually Thybulle or Nesmith), and salary filler. Ainge passed that up because he didn't want to make the trade before securing Hayward, but in order to secure Hayward he had to trade Bradley. Now obviously there is no real way to know where those picks would have ended up and maybe Hayward really wouldn't have been possible had he made that trade, but the reality is George was then and still is better than Hayward and I'd much rather have had George and Bradley vs. Hayward, Crowder, Langford, and Nesmith as George would have quite simply been the best player Boston had since KG (yes better than Irving).
And the I'm sure the Irving trade would have still been possible using the exact same assets with Bradley instead of Crowder. How different does Boston look with Irving, George, and Horford in their prime with Smart, Brown, and Tatum coming along than what we ended up with? That team very well might have made the Finals that first season, which hopefully would have encouraged George to re-sign in Boston (just as he did in OKC). If George re-upped, then I think Ainge would have been more inclined to move Brown for Leonard. Imagine that juggernaut. Irving, Leonard, George, Tatum, and Horford. Team would have been unstoppable. Might have only been a season, but still no way that team doesn't win the Finals and frankly even without Leonard, might have come out of the East and beaten the injured Warriors.
Lots of what ifs of course, but also a certainly plausible and realistic path.
As I've consistently stated, acquiring Irving wasn't the problem, the problem was acquiring Irving and then not pulling the trigger on Leonard. Why acquire a #2 (Irving), if you aren't going to pull the trigger when a #1 (Leonard) becomes available. If he was never going to acquire the #1, he never should have acquired the #2.