Ive lived in Cornwall for over 30 years. However I was brought up in a similar place in Devon, so there was very little no difference so its probably to be expected that I found it home.
I've seen people fit in and not. They all wanted to, so I'm not sure you can call who will and who won't. Depends what they miss from their old lives when they arrive. We have most things but they are almost all a decent drive away!
Do thunk about what you love doing on a day to day basis. Check out where these opportunities exist all year round. Some of the most beautiful have literally nothing in winter but the scenery - few neighbours due to second homes, and places close for the quiet season. But pick somewhere where there is more community and you'll have lots of interest groups, U3A, restaurants of which some open all year round, walks, doctors (few dentists and no NHS but that's the same all over I think), few gyms but sea swimming, running and hashing clubs.
If you think you'll need decent hospital access make sure you have good connections to Truro or Plymouth as its where the main hospitals are with little or no public transport to them.
If you are retired you could try coming for a month or two in winter and travelling around. I'd imagine many holiday lets would be happy to do you a deal for a longer stay over winter. You could stay in a few places....Cornwall isn't a small county!
Oh and wherever you buy.....make sure you have parking!!!