Paid extensions are allowed, which seems like a neat compromise.

If you need to add an extra API or something to the core to make your paid extension work, you can't charge for that, which I think is designed to incentivise "improve the extension API, contribute that back to the core project, then go wild on your commercial extension and see if you can get people to pay for it."

I have no clue whether this approach will turn out to work in the medium-to-long term, but it's a fascinating idea and seems at the very least like an experiment very much worth conducting.