This is not rocket science. I'm not talking about teams with no title hopes (Utah, Sacramento) that want to retain a star to keep fans in the seats. I'm talking about how the top teams build contenders.
You either draft an MVP-level player (Giannis, Doncic, Jokic) or trade for them and then build a complementary roster.
Masai Ujiri and Pat Riley have done it. Ainge did it once, but only because his former teammate Kevin McHale gifted him Kevin Garnett for garbage.
Ainge had so many opportunities to trade for the guy that would have put them over the top, but his obsession with hoarding draft picks has been a bust.
Just think rationally (I know you have it in you).
Just before the 2016 draft, they could have traded Jae Crowder and a couple of draft picks for Jimmy Butler.
Even if that meant not drafting Jaylen Brown, the 2017 playoff team would have had a much better chance of reaching the Finals.
The 2017-2018 team then doesn't need to trade for Kyrie Irving and sign Gordon Hayward to a bloated contract but adds Jayson Tatum. How does that team not reach the Finals?
In 2018-2019, they don't get poisoned by Kyrie Irving, and have the money to keep their core together of Butler, Tatum, Horford, Smart, Aaron Baynes, Terry Rozier, Daniel Theis and Marcus Morris.
Maybe this team doesn't beat the Bucks or the Raptors, but that core (minus Horford's loss in free agency) definitely beats Miami this year and is a much stronger match up against the Lakers.
In 2018-2019, Ainge could have gotten Kawhi Leonard but over-valued Jaylen Brown and his draft picks. Put Kawhi on that team and only the Bucks stop them from reaching a Finals where the Eastern Conference Champion would luck into the Warriors losing two superstars due to injury.
In 8 of the last 9 Finals, the championship has been won by LeBron, Kawhi, or Durant. The only exception was the 2015 Warriors who had the greatest shooting guard combo in history, plus Iguodala, the guy who could slow down LeBron.