I do not think that there is any immigration route for them.
There used to be a
Retired Person of Independent Means visa, but that was closed a while ago.
Do either of them have a grandparent who was born in the UK? An Ancestry visa may be an option, though that is a work-oriented visa. But I have not seen a requirement that somebody on an Ancestry visa must work in the UK.
EDIT: Overtaken by
CR001.
PS: There are other countries that do have visas catering towards people with an independent income, such as Portugal's D7 Visa, Spain’s Non Lucrative Visa, Greece's Financially Independent Person visa, French Long Stay Visa and Italy Elective Residency Visa. You may want to see if your parents qualify for one of those.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.