Yarn’s offline support is not directly tied to PNP. Yarn v1’s support for “offline” installs was a day one requirement, and as I understand it one of the primary drivers for Facebook engineers (at the time) to drive the creation of Yarn.

If offline installs is what you want, I don’t see any advantage of node_modules compared to this feature - only disadvantages (size, noise, and cross-platform incompatibilities).