If enough people subscribe to the idea, this might become the standard operating procedure and you won't even have the option of saying no.
It's the standard trick: the company will offer temporarily lower prices, then once enough people are onboard, increase them again.
The company sets a new normal, gets all your fitness data, the people get a temporary price reduction, and lose their privacy.
If they can prove you were cheating the fitness tracker they can deny the insurance claim when you die. Not great for any family members who were counting on that.
Health insurance is something that pays for things continuously while you are alive to keep you alive and healthy.
Live insurance pays your beneficiaries (usually people that depend on your salary) when you die.