We can't compare NBA championships in the 60's to SB victories in the present
There were only around 12 NBA teams back then
Consider today's NBA and the success of Lebron James over the past few years. Recently his only formidable competition has been the Warriors and Spurs.
But let's do a thought experiment.
Eliminate the bottom 14 teams in the league, immediately, and disperse the best players on the now-defunct teams to the remaining 16 teams.
Sure, Lebron might pick up another good teammate. But perhaps the Celtics also pick up Anthony Davis, the Raptors get Porzingus, the Bucks get Hassan Whiteside, the Pacers get Karl Anthony Towns, etc.
In a compacted league, Lebron has one less round of playoffs to get through, but is his road to a title easier?
He may suddenly face a non-zero number of worthy opponents in the East.
This thought experiment isn't the perfect analogy to a pre-expansion NBA, but it does make you think that a bigger league is not necessarily better. It might give you way more cupcakes.
Another way to think about it, in a 12 team league Russell faced Hall-of-Fame centers Wilt, Walt Bellamy, and Nate Thurmond. 3 of 11 opposing centers (27%) are Hall-of-Famers. How many teams does Lebron face with a hall-of-fame center? Maybe 1 of 31, Anthony Davis, 0.3%? How many teams does Lebron face with a hall-of-fame SF? Durrant, Kawhi, maybe Paul George, maybe Jimmy Butler, maybe Giannis Antetokounmpo. That 5 of 31, tops, 16%, being generous.
A 31 team league lets the few true superstars feast forever on cupcakes.