well, OP I don't believe for a minute that you are not still reading- either as a genuine poster or as a writer looking for info, so here goes.
I know about 4 couples- all women like me, in their 50s, with long marriages.
couple 1- have not had sex for 10 years. She feels no emotional intimacy, they lead pretty much separate lives, and she doesn't miss sex at all. As far as she knows he is faithful.
couple 2- married for yonks, he has a health issue- they have not had sex for years either. she is not bothered.
couple 3- married 25+ years again- have sex now and then, mainly when she can muster the enthusiasm as she feels she ought to, for his sake.
couple 4- maried 25+ years - have sex now and then-variying from 2 times a week to 2 times a month, to none for ages, had a drought with none for years as one of them had a health issue.
I read somewhere that 25% of marriages of 25 years plus are sexless. sexless defined as fewer than 10 sex sessions a year.
my own take on this is that it's whatever keeps everyone happy. I do not agree that one party is always just putting up with it. If you do not have sex your libido eventually shuts down- what you don't have you don't miss, in time.
also, by the time couples have been married for 25-30 yrs the disentangling of finances etc is simply too much so they stay put even if it's not an ideal relationship.