I work with a couple of women who have been engaged for several years with no plans for a wedding any time soon. And, I doubt that there will ever be a wedding.
These women are cared for by their mates and even have an engagement rings. And, if their mate should happen to die, they will receive some inheritance.
The advantage to this is that should the relationship fail, she gets to keep the ring and he removes her from the will, while retaining all of his possessions. In short, she has no legal recourse.
Such relationships have been around for thousands of years. In the Old Testament, these women were called concubines. They were, in a sense, wives with no legal status.
Sir Paul would have been better off with a mistress than a wife. Who knows, maybe she would have stayed if she know she wasn’t going to get any money.