The social norms are stupid and this would give you the excuse to ignore them without giving it a second thought (hey, the car is making me wear it!).
They should hold off on the warning until you've shifted the car out of park.
Maybe instead of a power button, cars should turn on when you buckle up after the brake pedal is depressed.
People will wear them or not. Making things more complex just means it will fail and make people do strange things.
If someone makes such a deliberate and conscious decision not to wear a seatbelt, maybe you could assume they have their own reason for that.
It's not even clear that forcing people to wear a seatbelt saves lives. It may increase the risk to other people, if people are seeking out a level of risk that they're comfortable with: https://en.wikipedia.org/wiki/Seat_belt#Risk_compensation
The evidence on either side is not enough to justify bankrupting fines.
The best the designers of products can do is "make them as safe as possible by default". There will always be people who actively work against those safety mechanisms though (like people turning off their ESP for that extra "sports-like" performance for their car - in urban traffic).
I don't really give a hoot about the counter arguments against it, but at the same time I wouldn't let anybody in a car I drive without the seatbelt on.