Here's the problem: Whoever trades for these players has to pay for them. . .and pay a lot more than they would for a rookie who has the very likely potential he will be better than Leonard. Look at the top 1-3 picks of just about every draft lately. Kyrie Irving, Kevin Durant, Unibrow, John Wall, etc.
Combine the fact that you barely have to pay for those players for years, and their value is a LOT higher. It isn't just the potential, which is greater.
As for last year's draft, it was notoriously poor and was expected to be weak going in. This one is the opposite.
The Cavs will not throw away their pick for a costly role player(s) and a guy who while good, has never made an all star team.