For the majority of women (and men) who post here, it's the case that they have been in difficult, abusive or just unhappy marriages for years before they post. These are people who have tried, tried and tried again to keep it together and they have come to the end of their rope, which is why they are posting. It's their way of seeking permission to make the break. As always, what you see on here is a distorted and self-selecting sample. People who are happy in their relationships aren't going to come on here and post about it - they're out there living their best lives.
It isn't 'more and more posters' advising women to leave their husbands, it's the same as it's ever been.
If you read what these women and men post, the first post is often a testing of the waters. Later on, a lot more will emerge, making it clear the relationship is much worse than it seemed at first sight. If you've been in a long, deteriorating marriage, you're also likely to have lost the boundaries of what is normal and be putting up with things that are unacceptable.
Some relationships can and should be fixed; I'm not normally a LTB person, except where there is addiction involved. But it's often the case, when you read further, that these relationships are deeply dysfunctional.
There are definitely a small handful of people who seem to cry 'LTB' at the drop of a hat, but it is very much not the majority.
Hope this is helpful to you; I've been on MN for 10 years.