Ainge had years of evidence that Terry was a mediocre and inconsistent backup who would cause chemistry problems if not given a full time starting role, and that when given a full time starting role, Terry played like an average at best starting point guard.
Roy is right -- Terry has done work on his game to improve as much as he has.
That said, Ainge had to have seen the potential for sustained improvement in Terry or else he wouldn't have drafted him.
The big sliding doors moment for the franchise, other than Hayward's ankle, is after the 2018 post-season run. What if Ainge had traded Kyrie at that point and gone all-in on the young'ns-plus-Horford? Hindsight is 20/20, of course.