I am sure there are more Mach3 users than pro system users, it was the only system that was available to small machine builders, home users, specialist developers etc. It has been adapted for many machines - mill, lathe, plasma, drag cutters, 3d printers and many more and has had new screens developed for it - how many pro systems can switch screens in seconds?
It has weaknesses, many of them, but i think it still stands up as an excellent system even in the face of offerings like linuxcnc which is gaining acceptance now.
Mach4 has a long way to go yet i feel.