Everytime a game releases a new version, it is going to lose some of its players - those who decide not to move on to the new version, but stick with the older version. it is a fact of life and will ALWAYS happen, regardless of the game.
The problem that WotC faces is that the 3.x version was released under the OGL, and that allowed other companies to take the ball and run with it. Other companies that have build up their own followings, and essentially taken their customers from the D&D pool of customers.
This makes attracting them back that much more difficult.
Attracting new players -- WotC (i.e. Hasbro) is one of the few companies with the resources to do this. Very few others can.
As for getting 4e to move to 5e - that depends on how they handle the actual revision. The best way, IMO, it to basically work on improving 4e in a variety of ways, rather than try to redesign things from the ground up. That way, it is more natural for 4e customers to move to 5e, as it is the "same game" only improved...
