Disclaimer: I have no Enfield experience.
It turns out that patent encumberance isn't the only thing that makes something difficult to make.
Many older weapon designs were optimized for low volume manufacturing by skilled machinists, and required hand fitting by gunsmiths and armorers. That made sense when human labor was cheap and skilled.
The Garand and M14 receivers, for instance, are very complicated to build. The 1911 is also a much loved design, but most 1911s are either built to loose tolerances or require custom, per-example fitting.
Comparatively, the AKM receiver is bent sheet metal. Any workshop that can do basic metal work can build an AKM; the barrel is the only specialized part.
The M4/AR15/M16/AR10 family of receivers were designed post-aerospace industry, and are made to be mass produced by machining down aluminum forgings. I know multiple people who have completed their own AR15 receivers on CNC equipment.
The SIG handguns manufactured in the USA are taken from billet to serial number in a single machining center; no operator intervention required.
It turns out that it can be very difficult to re-create old things. Often, the original tooling is missing. The techniques used may no longer be taught nor widely practiced.
Comparatively, building a modern mass produced firearm is a matter of having the right CAD files.