It isn’t simplistic at all. It suits people to believe that.
Having half a dose of your vaccine is risky. Having started, you might as well finish, or you might just be taking a wider variety of risks.
I don’t see the logic of the above when boosters are needed every twelve months or even every six months. If someone is worried about something, why would they take repeated doses?
I think some people are more risk adverse. I have ne er taken illegal drugs. When I was younger, I didn’t ‘try’ drugs. I never took a pill/a popper/heroin/coke/magic mushrooms. I’m not comparing the vaccine to any of these but using as an example of how worried I would be about the risks involved. I wouldn’t ride a motor bike as the risks are higher than cars. Some people are happy to try new things, the payout is worth the risks. We are all different.