I really don't agree with the assertion that the NBA's talent is spread too thin. There are many, many more good players today than there were in the 50s or 60s. Expansion of the game stateside, as well as into Europe, Asia, Africa, and South America means that the pool of available good players is much, much larger than it was in previous decades.
The big difference today is that it is much harder for players to stand out today than it has been in the past. With a higher level of all-around talent and with modern weight-training and vitamin supplements, athletes that reach the pro level are able to maximize their talent much more easily. I think this creates a plateau affect, where you get more players who are better than ever before, but fewer players that really can distinguish themselves from the rest of the pack.
On the coaching and GM front, modern schemes and computer analysis make these jobs much harder today, at least in terms of sorting through the various pieces of data available. I would be willing to bet that you give a mediocre GM or coach from today's NBA the knowledge and technology they have available to them now, and put them in the 70s or 80s, and they would absolutely dominate the compe ion. THe problem, again, is that everyone has that data available now, so it's much harder to distinguish yourself.