Is this really true, though? Look at the playoff contenders in the past few years. None of them were truly built through stockpiling high draft picks:
Warriors: Curry at 7th was the highest pick among their core. Thompson at 11th and Green was a second rounder. KD via free agency.
Spurs: Have not had a lottery pick in forever. Kawhi was a 15th pick and he was acquired on draft day.
Rockets: Harden was traded for. And Howard was signed via free agency. And now Cp3, who was acquired via trade. The only draftee in their starting lineup was a late 1st round pick (Capela).
Cavaliers: Well, you can say that LeBron was drafted by them but he was a free agent signing in his second stint as a Cav. Kyrie then became the Robin. Of course, they had two more number 1s but they used them as assets to trade for Kevin Love.
Celtics: Their mini rebuild after the Big3 era didn't last long as they were back to contending in a hurry. Who led them back to the playoffs? Some guy they acquired via trade, IT. They also signed free agent Al Horford. Now of couse, they have Kyrie.
Raptors: DeRozan was drafted 9th. Then they acquired late-bloomer Kyle Lowry.
The only argument you can make for tanking for high picks is the late 2000s and early 2010s Thunder. So yeah, I don't believe getting high picks is the only way to get better in the NBA.
This argument is hard to make. Let's use lottery picks the team had in the last 10 years whether acquired via trade or traded away (because you have to include the trade value of having an early pick):
Warriors: 4 (2012, 2011, 2009, 2008)
Spurs: 0
Rockets: 4 (2013, 2012, 2011, 2010)
Cavaliers: 5 (2014, 2013, 2012, 2011, 2011) They also used the trade value of 2018.
Celtics: 4 (2017, 2016, 2014, 2013)
Raptors: 6 (2016, 2013, 2012, 2011, 2010, 2009)
Portland: 5 (2017, 2013, 2012, 2012, 2008)
Now let's look at players who were lottery picks, so we can see the talent irregardless of how it came to the team:
Warriors: 5 (Curry, Thompson, Durant, Livingston, Iguodala)
Spurs: 3 (Aldridge, Gasol, Gay)
Rockets: 5 (Gordon, Harden, Paul, Johnson, Nene)
Cavaliers: 4 (James, Thompson, Green, Love)
Celtics: 8 (Kyrie, Brown, Tatum, Smart, Hayward, Horford, Monroe, Morris)
Raptors: 3 (Pöltl, DeRozan, Valanciunas)
TrailBlazers: 8 (McCollum, Lillard, Collins, Aminu, Davis, Leonard, Papagiannis, Turner)
Also the Wizards are another example of a team that built via the draft, but I didn't include them even though I should have.
Essentially, lottery picks are where most of the elite talent comes from. There are, of course, exceptions (Draymond, Giannis, Kawhi - although the latter two were 15th overall), but most of the elite talent in the league are early draft picks. Hence the value, whether used via trade or via the draft. Obviously not all these players listed are critical to the team, but you can see that a lot of talent is in the lottery hence why it makes sense to tank.
Or if you want to look at this another way, you can look at players in the lottery who have made all NBA teams vs those out. Note, this doesn't include guys who just made the All Star, not all NBA such as Beal and Hayward. I'm starting earlier cause that's where a lot of still elite players are:
2003
In: James, Anthony, Wade, Bosh (4)
Out: N/A (0)
2004
In: Howard (1)
Out: Jefferson (1)
2005
In: Bogut, Williams, Paul, Bynum (4)
Out: Lee (1)
2006
In: Aldridge, Roy (2)
Out: Rondo (1)
2007
In: Durant, Horford, Noah (3)
Out: Gasol (1)
2008
In: Rose, Westbrook, Love (3)
Out: Jordan, Dragic (2)
2009
In: Griffin, Harden, Curry, DeRozan (4)
Out: N/A (0)
2010
In: Wall, Cousins, George (3)
Out: N/A (0)
2011
In: Irving, Thompson (2)
Out: Kawhi, Butler, IT (3)
2012
In: Davis, Lillard, Drummond (3)
Out: Draymond (1)
2013
In: N/A (0)
Out: Gobert, Giannis (2)
2014-17: None (0/0)
Total
In: 29
Out: 12