NT4 started with a kernel mode, user mode, security model and drivers had to be written and validated accordingly.

9x, me, and even compatibility parts of XP (up to some service patch IIRC? Might have been SP2) would still allow dos mode realtime BS for any driver that wanted.

I loath all the dang software modems too cheep to ship a decent device in a single unit and instead slice off the user's already constrained resources.