MixW is still around and available. I still use it. MixW costs money to register ($70 for MixW3 I think) while DM780 is free - the likely reason that MixW has dropped out of the mainstream. Hams will hand over $10,000 for a new radio but think software should be free
MixW has not really been updated for some time too. Lot of alternative options today for software too and some are much better too.