By the time Windows won't run existing VB6 programs Windows will have changed a lot. Even now you have to move to a 64-bit version of Windows before most old DOS and Win3.x programs won't run.

I expect VB6 to still be supported in the next Windows as much as it is in Win7-64, i.e. a little clunky but still possible. After that I'd like to think most of us will have moved on.

Windows 7 Pro Lifecycle