Isn it in WotCs interest to make a game that is as incompatible as possible with previous material to force people to buy a whole new game?
No. In fact, it is against WotC’s best interests to make a game so incompatible that the majority of existing players would perceive it to be a “new game”. Part of the value of DUNGEONS & DRAGONS is related to the number of people who know how to play the game. That community of existing players is a valuable resource that no other RPG in the world can draw on. We want you to be able to take your 3rd Edition character to your local game convention, sit down at any D&D game being played, and be able to join right in with a modicum of disruption. Rendering 3rd Edition totally incompatible with the games that the existing network of players enjoy would not support that objective.