I think most of what you’re describing as slow fading is just growing apart - a natural response to no longer having as much in common or no longer enjoying the differences between you. If you don’t find it enjoyable, of course you’re going to be less likely to spend time with them. Less likely to say yes if they suggest something, less likely to invite them to something. I don’t think there’s anything wrong with that and I’m not sure why others do. It’s bizarre of people to suggest you should spend time with people when you’ve stopped liking them just because they still like you or want to use you.
But the depth and length of your friendship with someone should have a significant impact on that process of growing apart. A long, deep friendship deserves bit of investment - either a holding pattern for a while to see if it’s just a temporary blip or some communication to see if you can set things right.
I don’t think a dramatic showdown is a likely to work for many people, but saying something along the lines of - “Barbie, I know Ken was a total dick to you but I’m not up for spending all our time talking about him. Can we go to a bar and not mention him for an evening?” Or “Little old lady who lived in a shoe, I do realise motherhood has changed your life, but all you ever talk about nowadays is your kids. What happened to the vivacious woman who went treking in Peru, won the WI marmalade trophy 8 years running and organized our big reunion 4 years ago? Can we please talk about something else some of the time?” Or “Madonna, I’m going to say no again. I’m kind of over the whole club scene. Would love to meet up for Sunday lunch at a pub or something, though.”
(Very much agree it’s good to have boundaries about not being used as a therapist. It’s not much of a friendship if you never talk about anything negative, but friends wallowing in a negative experience again and again is bad for them and you).