Dude
I highly doubt your conspiracy theory regarding women. It isn't that they "know that LTR's don't work", the problem is that they don't know what it is they want at all.
The problem with marriage remains the expectations people have with it. Getting married too early and not experiencing life is another problem as well. It is a fact that marriages after the age of 30 have a significantly lower divorce rate, it's all about living in your twenties and finally maturing before taking the big step.
another thing to think about is the statement
"50% of marriages fail
Of the 50% that stay together, which percentage are happy? I'd say, not a huge chunk.
So the prospects are very bleak."
This is partially true, there are many unhappy marriages, but there are also many divorces that didn't need to happen either. It's a fact that in a LTR (I'm talking over 30 years lets say) that people will fall in and out of love several times. There will be difficult times, in this day and age people bail out too soon when the first hint of trouble happens when they may have been able to work it out. Granted, some relationships didn't have a hope in hell from the beginning, but some could have made it with some effort.
Nothing worth having comes without a struggle, the same with relationships. I know several married couples that almost called it quits, and actually admit "I couldn't stand him/her anymore, I wanted out", but stayed together, got through it, and are now happier than they have ever been. We have become a society of quitters unfortunately. A LTR or marriage is what you make of it, no more, no less.