Thanks for explaining that. I forget the exact history, but I know they've changed up the extension types/APIs multiple times in the past few years. I think a lot of developers/companies gave up on them after being forced to rewrite.
To confirm, is the model you and the parent would like:
"After x units are sold, or after 12 months (whichever comes first), the project will be open sourced and changed to free in the App Store. It will then be supported with an optional in-app-purchase donation - with no nagging or feature differences."
I paid money for an extension that promptly became unsupported and then died in an upgrade. Which is fine, whatever, but it did put a big question mark over the category of "paid browser extension" for me.
Knowing the source will be out there would fix that part for me - I may not be able to take over dev, but I could at least try to fix things that break until I find a replacement.