Earlier quoted context omitted.
When you train an updated model on a new dataset do you really start by deleting all of the data that you collected for training the previous version?
Probably not. But if it’s the new data providing the advantage then you’re not exactly better off having the old data and the model vs. just having the model.
As opposed to not being able to fork it at all because an "open source" model actually just means "you are allowed to use this particular release of our mystery box."