People can only grow so tall and the other positions keep getting taller and taller thus limiting the dominance that "Big" men can have.
Being healthy has become another issue. The fact is joints (especially the knees) are not made to handle that kind of physical activity and constant abuse that comes from playing sports at a high level let alone the exponential stress that comes from being 7 feet tall.
All that being said "Big" men still very much have a place in this league as we saw from the Pacers this year.
Another interesting question may be will we ever see the dominance of another AI sized guy?
If the league were to go into a recession such as what Boxing's current state is then in a talent devoid era Big men could become dominant again.
I find this highly unlikely as not many sports love tall guys quite like the NBA.
Boxing had horrific corruption, stupid title rules, people becoming aware of the brain damage done by not being Tyson, and the MMA poaching talent all working against it to create what it is today. Anyone over 200 lbs and a pair of gloves has a shot at being the heavy weight champ.
NBA does not pose a lot of serious long term health risks that people care about (head trauma) to have that be an issue and other than another basketball league becoming more popular than the NBA they will not have to compete for athletes with another sport.
I actually expect the NBA and basketball in general to grow huge when people realize (10-30 years will show the effects) that there is almost nothing you can do to reduce the head trauma of modern football (even if they banned PEDs since grade school). Basketball is already world wide and with soccer being the world's sport that anyone can play with little risk of injury and minimal equipment required NFL style football will never catch on world wide.