You could also look at the Spurs and say they tanked for Timmy and have been the best franchise in the league for the past 15 years
There is one key factor with this scenario...
1) The Spurs already had a superstar and one of the greatest centers every to play when they drafted Duncan - most lottery teams do not have that luxury
2) Throughout this era the Spurs have had Poppovich - one of the greatest coaches of the era. Again, a luxury most lottery teams don't get.
3) The A player like Duncan comes along once a decade, typically. The Spurs were incredibly lucky that the season Duncan was coming in to the draft happened to also be a season they really sucked in. When you know you have a Duncan / Lebron / Jordan / Anthony Davis caliber talent coming up in the draft, then tanking is worth the risk.
95% of teams who tank don't have a superstar player, a super coach or the certainty of a once-in-a-generation talent in the upcoming draft. That changes everything - for the Spurs the stars aligned, so to speak.
The later drafts (Parker and Ginobilli) came down to great management and decision making, so you can credit them for that - but all of that would have meant nothing if they didn't get Duncan.
The Denver Nuggets are another example of a team that tried to build around glorified role players. After several 50 win seasons, they are now 29-50.
Only after the Melo and then Melo/AI versions of Denver failed to get out of the first round.
FYI,
They actually made the Western Conference finals and lost to the Lakers (it was the year KG was injured for us).
The Lakers beat them 4-2 and then went on to embarrass the Magic 4-1.
But I get what you're saying.
Ultimately to win an NBA championship you've got to get lucky in the draft, or you've got to get lucky in free agency.
Both require insane luck.
The difference is, in trades and free agency it's not all luck. You do have SOME control. You have the ability to at least put your best foot forward. You can talk to free agents, try to convince them that your team is where they want to be. You can talk to teams, try to tell them that your package is what they need in return for their star player. In the draft lottery your team has zero control over those ping pong balls. How many times has Cleveland taken the #1 pick despite not having the worst record? In some cases they didn't even have CLOSE to the worst record. How many teams have tanked to the point of utter disgrace yet still missed out on the top couple of picks?
Tanking and building through the draft is, for the most part, like betting on Bingo or playing the lottery. You can control how many lottery tickets you buy or how many numbers / lines you play in order to increase your chances of winning...but at the end of the day it's out of your hands, and you're more likely to NOT get the #1 pick then you are to get it. Plus every moment of tanking hurts your reputation as an organisation and hurts the attitude of your team. Trying to get players through trades an free agency, and being beaten to the deal, doesn't have that affect - at least you are seen as trying to make an effort to get better.