I agree with you about valuing the ideal of two people loving each other for a lifetime, sharing memories, developing a level of trust and love over decades.
I realize I have some cynicism, but I know that marriages can last for reasons other than love. A big reason marriages lasted a lifetime before women had equality is because it was nearly impossible for a woman to survive outside marriage. A person will tolerate a lot when it is their primary option. It made even bad marriages a better option than being alone and struggling to make money in a world that didn't hire women, especially professionally. Also, when the dynamic is unequal then conflicts are resolved more readily because there isn't full discussion or two viewpoints, so no conflict. I also know marriage where the woman is in control and the man so beat down that he doesn't have the confidence to leave. Imbalance of power dynamics creates lasting, but destructive dynamics because the dominant one has everything just as they desire it, and the submissive one is too sick, tired, and beat down to go through the difficult process of leaving.
Marriages can also last out of force of habit. When lifestyle habits are established over years, it is incredibly difficult to break these. When people never consider making a change because routines are set, then they don't even when the current habits are not in the best interest of either party. One example that comes to mind involves sexuality. I recently read that 20% of marriages are considered sexless (meaning sex 10 or less times a year). If that is the choice of both partners, then great, but when it is not, then that is a horrible oppression for the partner who needs sexual expression and is committed to a monogamous marriage. I have seen that dynamic a few times over the years and for the partner left longing, it can dismantle them psychologically and even cause physical ailments to be exacerbated. I've had friends develop more severe chronic fatigue and pain when left without physical, sexual expression. Sexuality runs to the core of our psyche (which society never talks about), and so when people are incompatible sexually for any reason including libido, it can destroy one or both partners. If one partner has low libido and the other successfully pressures them with psychological coercion, then that also has destructive results. Sexual monogamy can involve amazing deep trust and connection, but it is amazing how much damage it can cause people. Yet many live a lifetime in a dynamic that destroys them to their core, even when everything else in the relationship is a peaceful friendship. I never had any comprehension about the complexity and importance of sexuality in what I was taught when young. I was always taught it just fell into place if you were kind, unselfish and available as a true friend. That is an absolute lie. Sexuality has a life of its own, and even though it can represent relationship dynamics, it can also represent physiology and previous life experience of both partners.