I think the finger really needs to be pointed at the people who fitted GX16-type connectors in the first place - the spindle manufacturers. Looking at spec sheets, these do seem to be nominally rated at 5A, although quite how "hard" a limit that is is open to discussion. I'm not sure that the connectors are capable of even taking a cable that is nominally rated for 10A.

In practice, in the amateur machines where these spindles are typically used, I doubt that they often are used for continuous currents like this. Attention to proper cable management and avoiding flex in the connectors is probably a bigger issue. My own spindle and HY VFD are now in use in a second router and are about 7 years old, still going strong. In fact, outlived the PC and monitor that drive the machine, both of which have needed replacement in that time...