Not trading for AD would only be a disaster if he set the team up with only one path forward (getting AD). But if no AD trade materializes (due to NOP asking for too much, AD saying he wouldn't re-sign here, or NO deciding to keep him), we're still in a great position. Yes, AD and Kyrie is the fastest and surest path to contention (but still not guaranteed), but even if neither of them is on the team next year we'll have a great base to build off of.
Not getting everything you want isn't a disaster unless you are 100% relying on it, and if Danny's time here has shown us anything it's that he always has a plan B (and C, and D, and E...)
Honestly, this reminds me of how people were mad that Danny wasn't sending the #3 pick in 2016 for Jimmy Butler. Or calling for his head when he traded the #1 pick for the #3 pick plus the Lakers/Kings pick. Or calling Danny incompetent because he didn't trade everything for whatever star was available at time (PG, Boogie, Kawhi, etc.). You don't fire one of the best GMs in the NBA just because some fans didn't get what they wanted.