I do think some people are being quite mean towards those seeking out a different lifestyle on here. A lot of the people criticised on this thread sound like they have enjoyed their new lives - it's just the very late stages that are problematic. But let's face it, that stage isn't necessarily easy no matter where you live and people can feel isolated and lonely in the city just as much as rurally.
Also, there are actual people living in the countryside! Are people born and brought up in the country to be scoffed at when they get elderly? Are they expected to suddenly move to the city too?
Perhaps it's also worth questioning why it is people want the dream of the rural idyll so much if city living is as wonderful as people describe on here. Cities are noisy, polluted and busy, not great for older lungs. Property costs are often very high. People don't always develop close networks or know their neighbours. Look at a lot of the threads on here where people prefer not to know or interact with anyone living around them. A lot of the benefits of the city are expensive for many and to live with a garden and to get the benefits of peacefulness and nature is out of reach for many in the city unless they are very well off.
We all have one life. If people don't pursue their dream when they get the chance, and for many this is when their children are grown and they finally retire, when can they? Are they supposed to live in an area they don't love forever just in case?
The part that's wrong is putting pressure on your grown up children bit. If a grown up child moves to Australia, they cannot get on the phone all the time pressurising for parents to come visit. If someone pursues a new life or moves far away, they do need to sort out their own networks and support system and eventual care, or decide on a plan for that if need be.
This case sounds like the parents themselves don't agree on where they want to live and the lifestyle they want. Perhaps the OP has to stand back completely and let them sort it out. They are grownups and it is their lives. In the meantime she needs to not allow herself to be pressurised for more stays as she obviously sees them loads already and she deserves to not be endlessly travelling and hosting and be able to enjoy her own time off. It isn't fair on her to pressure. I expect this isn't just the rural thing however, but failing health and over-dependence on your partner and no wider network of friends can do this too. People can fall into this rut sometimes when they get older. How can we encourage people and ourselves to keep looking outwards and keep making new connections for ourselves, not just to rely on other generations who might not have the same time or interests, to provide interest, chat, companionship?