You'd be forking 2 year old code, he hasn't updated the repo in the last 2 years, knowing that the next OS release would leave commercial users at his mercy. Apple also doesn't just hand out kernel extension signing certificates to everyone that signs up for a developer account, you'd have to be someone with some reputation.
It was a well-executed fake punt, he'll get people to pay; Catalina is out now and if you want to pick it up from the fork it would take a long time to get it right. That said, I would imagine organizations like Google will make the effort after paying the toll this time because the price will only go up.
It all sounds clever but obviously Catalina was not released yesterday, it was released some time ago and if a companies software required changes for Catalina they should have noticed many months ago while testing on the beta releases.
Problem is that the 2017 version, the last version that's open source, won't run on Catalina (or Mojave or High Sierra for that matter). The amount of fixes and compatibility patches that the author did is quite extensive. Replicating his work would be quite an undertaking and wouldn't be trivial.
