That would require research beyond my interests.
But if it helps, only the Bulls, Lakers, Celtics, Rockets, Warriors, Pistons, Mavericks, Spurs, and Heat won titles between 1980 and 2015. That’s a LOT of recent repeating.
But there isn’t any way to prove what’s harder. But right now, the NBA has 30 teams, which means the best 360 players make the NBA. In an 16 team league, only the best 192 make the league. Stands to reason talent is more concentrated, what without the worst 168 players hanging around....