I'm just concerned 30-45 win seasons with an ever-changing cast of scrappy / hard-working / underdog / buy-low-asset / borderline-starter type players is what we have to look forward to for the next 4-5 years.
That's a bit unfair 
We all know DA is not planning for this to happen.
Building a contender is not sthg you can plan doing step-by-step as if you were playing a video game, smoothly advancing from one level to the next.
We do need to get lucky at some point.
Umm, yes it is - look at how Bird put the Pacers together. How would you rather do it?
To be fair, the Pacers got pretty lucky drafting Paul George with the #10 pick.
The Pacers are no longer a contender

They were one two seasons ago, but even then, they were much behind MIA and SAS.
Anyway, the point here is not about the Pacers - and incidentally I do like a lot Bird's work as a GM.
Point is that you need to get a star. Stars arguably have more impact in bball than in any other team sport.
And since each and every of the other 30 teams is looking for a star too, you'll need some luck: there are simply not enough high impact players for everyone.
IMO in every other aspect of the rebuild planning is important: getting a good coach, building a reliable roster, scouting and drafting players.
But acquiring a star is more like hunting: you have to wait patiently until the right opportunity arises, and then you have to be ready to respond instantly.
Ainge was ready to seize the moment if the opportunity appeared this year; it simply did not happen. Next year we might be more lucky; or maybe not; this is something outside our control.
Hence, other than remaining flexible and patient, there is not much we can do about it.