Counterpoint is you can already legally build your own gun legally as long as it abides state and federal laws. These kits offer a safer diy experience than just throwing a 3d printed Glock clone together and hoping it works. Plenty of people have also built AR15 variants from "80% lowers" which require some diy milling and assembly. Having safe and tested diy options is likely a good thing for overall safety.
Supreme court doubtfully even needs to rule on this, Heller covers this already as you said. This won't stick.