Then every customer would have a financial incentive to always lie and say they completed their goals.

It's already iffy with the 'lose money if you don't meet them' approach, but if you could actually get a piece of all the users moneys, yeah, you might as well say you're a winner (or only create goals where you can easily win) and game the system.

That's the hard part of the problem, and the bit that makes it worth solving.

You know how the customers who pay the least are the most work? Well this idea creates customers who literally pay a negative amount. My sense from the erstwhile GymPact -- https://blog.beeminder.com/gympact/ -- is that those "customers" tended to be a nightmare.

Hence Beeminder rationalizing just keeping all the money! :)