I am the developer of this extension. It has a few thousand users. It is and always has been entirely free to use. Occasionally I get emails from people who need help using it or who have found bugs that I need to fix. For this reason I need to be able to open Sketchup to use, test, and develop my extension. I have no other reason to use Sketchup as I am not a designer or architect, so I have no personal need for a license.
It seems that if I want to ensure that my extension works correctly in Sketchup 2018 I have to buy a license. This does not seem fair. Maintaining my extension is something that Sketchup and its users benefit from, not me. Is there any workaround? A license for extension developers perhaps?