> Framing this product in terms of time spent rather than value created gives rise to all the wrong incentives

Mostly agreed, however, when you leave the determination of the value created in someone else's hands, other wrong incentives come up. And... the value is often reliant on execution of other areas of business outside your control (support, marketing, pricing, etc). I can do the same project for 2 companies - one will struggle to get any value from it, another will get millions.