I thought it was always 'forever.' If it were 'till the feelings of love go away,' everyone would be divorced within 7 years at best.
You're right, most people in their 40s are divorced or remarried these days. It's rare to find someone who's been married over 20 years.
I think that 'forever' is good in that it gives you a strong foundation, and stability for children. It also decreases the spread of STDs when partners remain faithful. However, this is a bit too idealistic in today's world, because there is so much temptation, we travel and meet so many new people, and people change, and we live a lot longer. It can often be more of a jail sentence than 'happily ever after.'
With the introduction of the idea that 2 people of the same sex can now marry, I think that marriage as an institution is on the way out, to be replaced by short term mutual commitments and living together. But this will bring irresponsibility on the part of fathers, because men are biologically inclined to 'spread their seed far and wide' and without any moral or marital restrictions, could wreak havoc on society.
It's a Catch 22: Ideally, committed marriage is necessary for the stability of the next generation. But realistically, 'forever' is a long time to stay committed to someone, as feelings don't last that long, and people are narcissists, loving that "in love" euphoria!