A thread like this here on MN is not going to reveal the proportion of people who settle. Someone else has asked why that is. They wondered why would a person would be more likely to rush on to gush about their relationship than rush on to admit it if they felt they'd settled. It's not rocket science. Settling is a negative thing in most people's eyes (You can tell that just from all the horrified denials) Settling isn't something you look forward to for years, or strive for, or hope for for your children. It could be seen by some as an unkind thing as well - to be secretly putting up with a person who you don't love very much for years just because you can't face leaving. Of course everyone is more likely to post if they can talk about how the sun shines out of their spouses arse rather than any massive mistake they feel they made and are STILL ploughing ahead with.
And those saying settling is rare, or is actually a 'proper' relationship, or doesn't happen these days, or is just another way of saying you've grown up ... well ... i would disagree. I think you can probably only recognise real settling if you're personally doing it. Or have done it. Or someone very close to you is doing it.
I've done it. I did it for 14 years. I'm not proud of it. I left. Now i finally have a sunshine arsed relationship of my own which i can and will gush about now .... That 'unsustainable' strong sexual physical passion and the butterflies in the tummy is all just as fierce for us after 12 years of mad shagging. And yes that's along with the hard graft: the often mentioned hand holding in A&E, pregnancies, the losses, the new parenthood, the (now) aging parents, the teenagers and the finances and the cups of tea. Funnily enough i went through most of that last lot in the settled relationship as well. It's no particular measure of a relationship for me. You can actually go through all that with someone at the same time as NOT being particularly happy. Just like you can get up every day and go to work and do a job well even if you're not enjoying it
Anyway my point is - would i have been keen to post here 15 years ago and admit to my horrible settled state? No, probably not. I wouldn't have wanted to discuss it. Now i'm happy, yes, i can talk about it.