Unfortunately, Cuban seems to forget that it's not the NCAA which enacted the one-and-done rule, forcing high school graduates to spend at least one year waiting to join the NBA. It's the NBA who made that rule.
The league got tired of players coming in too early and sullying up the quality of the draft, so they're forcing them to spend some more time developing before they can enter the NBA.
For most players, that development occurs in the NCAA. Most players go to college, often for a year if they're an elite NBA prospect, and then they declare their eligibility for the NBA Draft.
Cuban thinks that a one-year stint in the D-League would be more beneficial, and would help them avoid the stranglehold of the NCAA and their ridiculous rules about amateurism and the related restrictions.
While Cuban's idea holds some merit, there are also a lot of drawbacks to the D-League.
Right now, the best pre-NBA talent is in college. To play in the D-League would be a step down at the moment, and players wouldn't really be more prepared, nor would they have a better resumé to present to NBA teams come draft time.
If a large group of players decided to go that route, it might be different. There would be a talent influx into the D-League, and everything could change. Also, the players would be paid, albeit not terribly well, so they wouldn't have to worry about the NCAA's oppressive rules about what they can do to earn money.
The elite players could also pick up endorsement deals for themselves, which companies would be glad to hand out based on future potential.
There's definitely a lot of advantages to this idea. But the blame shouldn't land solely on the NCAA. The NBA is in bed with them as much as anyone. The NBA is using the NCAA as their risk-free minor league system, and it's all under the guise of education.