Frank, 1) I would be offended if someone starting telling me how to live in retirement or 'advising' as you say. I would feel it was none of their business unless I specifically asked them what their opinion was.
2) When people have lived in the same place for a long time. Not only do they have a circle of friends, acquaintances and activities locally that they do not lightly cast aside they have a whole web of contacts, service providers from plumbers to hairdressers who they know and rely on. This kind of network is not easily replaced and in the new place you have moved to, knowing no one, you have to start building this network again from scratch. Many people do not want to.
3) Many people would prefer to live in a smaller property in a big city, particularly London than live in the country. Someone I know has said that the moment their DH dies they are putting their current village home on the market and moving as far into central London as they can, no matter how small the flat so that they can have the museums, art galleries, concerts and theatres on their doorstep.
4) One of my friends did exactly what you recommend and was miserably unhappy, despite being the most social of people she never settled in her new location and died of cancer within 5 years, I feel the cancer was exacerbated by her misery in her new home and the knowledge that having moved somewhere with cheaper housing she could not afford to move back.