My response:
I’m all for monetizing FOSS work but
The idea is that the package will no longer be available on NPM
This sounds like a supply chain attack to me(see colorsjs0). What you are proposing will require devs to yank packages off NPM and republish on the PayDevs registry, which will break builds globally.
A less damaging option(and maybe more attractive one too), would be encouraging devs to publish only AGPL/GPL packages on the public NPM registry and selling more permissive licenses using your implementation.
Regards,
Aravinth Manivannan