I'm a bit agnostic on the topic.
Commitments are built on a mutual trust, a promise to always try, and compromise.
Compromise can only go so far. Some people have some convictions that they could never compromise.
Once a point is reached where one of the partners is forced to compromise on something that would betray his/her convictions, then the relationship is over (whether they realize it or not).
For those for whom this point is never reached, a long and happy life-long love is accomplished.
For those for whom this point is reached, they would need to try again with someone else, or continue trying and be unhappy.
Yes, I am saying that the more un-betrayable-convictions one has, the more difficult it is for that person to remain in a long-term relationship.
A good friend of mine recently asked me the following set of questions: How do you rate your relationship? Now, do you think that there could be someone out there with whom you could have a relationship that you would rate higher?
My response was that as high as my rating of my current relationship is, it is based greatly on the quantity of time we have spent together (which is HUGE!). There may potentially be something that could grow to become better, but I would have to start somewhere lower and rebuild. The longer I stay in this relationship, the lesser and lesser possible it is for me to grow to be happier in another. (But it doesn't become impossible.) Time is critical. And the decision to leave a well-functioning relationship isn't something I'd take lightly or do on a whim. Good reason - such as, but not exclusively, reaching a tipping point - would be necessary for me.
Which brings me to the issue of "settling"... After so many years in a relationship, it's hard to avoid questioning your own motives for remaining in the relationship. Another attribute of relationships is that they evolve: the way a couple is when they begin seeing each other is nothing like the bond that is built after several years. In a well-functioning relationship, comparison of how the relationship is today with how it was when it began is a fun exercise, but serves incompletely as an indicator of potential long-term success. Perhaps the tipping point has just not yet been reached. Of greater consideration is the fact that the reasons why one enters into a relationship are not the same reasons why one stays.
So, is it inertia or is it "life-long love"? Are they the same thing? When a relationship lasts long, is it because they had few un-betrayable-convictions and willingly compromised at every turn (i.e. they let go of all of those things that made them individuals)? Or were they able to remain who they were and were fortunate to have found someone to share that with? I think these are difficult questions.