Everyone would just decline because there's no incentive to agree, and their product would die. Plus it would annoy users and cause stress.

Is that our problem? If a product relies on dark pattern acceptance of being "spied upon" to succeed, why should we want it to succeed?

Not necessarily. I've had Google Voice (And maybe iphone?) ask me if they can use my voice-mail to improve transcription data, and I always say yes. But it's an explicit ask for each time (If I remember correctly), which is nice.

You underestimate how many people are willing to take altruistic actions when the (perceived) cost is sufficiently small.

The main downside is an extra question during an onboarding flow.