When they retire, their lives will be less stressful and they will have more time on their hands, so they will find their role as landlords easier. Giving up paid jobs and giving up being landlords all at the same time when only in their 50s could be quite bad for their mental health, as it would probably feel like giving up a large part of their identity, leaving a massive hole in their lives. They need time to develop other hobbies and interests first, not just suddenly drop everything as though waiting for death!
As for selling - chances are they will be selling to other landlords who will be paying far more for the houses than they did over 20 years ago, so the new owners will probably then increase the tenants' rent. How is that a good thing? Also, if they are good and responsible landlords, why mess up their tenants' lives by chucking them out of their homes so that they can make a profit from a sale? I'm sure their tenants would far rather a bit of stability and experienced, reasonable and trustworthy landlords, thank you very much. It's the landlords with a cavalier attitude who are only in it for investment purposes and don't care about the human side, like stability, comfort and security for their tenants, or about selling up at short notice, that give landlords a bad name.
Also, these houses may be the couple's pension. You wouldn't cash in your entire pension on retirement as a lump sum, would you?
Also, do this couple have any children?
Personally, I can think of plenty of reasons why this couple wouldn't suddenly sell all their houses so that they can sit on a pile of cash in the bank which is worth less and less every year for the next 40 years of their lives, until they run out 10 years before their actual death...