I don't think MS particularly wanted to enter security market, they cared far more about making sure that all Windows computers had some level of baseline secur

I don't think MS particularly wanted to enter security market, they cared far more about making sure that all Windows computers had some level of baseline security solution. The constant virus epidemics were hurting their reputation and they wanted to address that; improving Windows security was pretty major theme for early 00s MS. Indeed I imagine especially then they would have been happy to be completely rid of the AV market instead of entering it as a player. The Vista era Defender product was extremely rudimentary, it's almost certain that the major commercial offerings were doing far more intrusive and complex stuff in kernel. Thus I don't think simply forcing everyone to downgrade to Defenders level would have made anyone happy. Calling MS lazy for not coming up with new novel set of APIs overnight is pretty disingenuous.