Developed by FrontCode Technologies, WinMX had become one of the premier P2P networks at its prime. After Napster and Scour were eliminated from file sharing, WinMX continued to take up the slack. Now it seems that its heyday is finally over.

The WinMX PNP Network, the FrontCode.com homepage, and the WinMX.com homepage are all offline. On September 13, 2005, seven P2P firms (including WinMX) were served with a letter from the RIAA ordering them to cease infringing operations immediately.

However, this may not be the final demise of WinMX, because if you do a DNS whois for "WinMX.com", the owner is now "WinMX Technology Associates" instead of FrontCode Technologies, as it was before. It may be back in some form or another.

Source

Yeh, likely as a "pay to use" service like napster