Great question. I am certain that when the majority of people say, "I do," they mean it for a lifetime.
What's missing is education about what comes after the wedding, the work needed, communication tools, the crashing of family systems, and so on. There are usually six people in the marriage: the two of us and our four parents.
We have to create a marriage that is you and me and the third entity, our marriage. The question is, "What's good for our marriage?" needs to supersede, "What's good or important for me?"
Most marriages can be worked on and make it with growth and secure attachment in other words, happy, loving, content, sexual, fulfilling.
But not by leaving things alone, sweeping topics under the rug, or taking each other for granted.
Many people put tons of effort into their education and career and are very successful. Use that same skill set to educate yourselves relationally.
Check out the Connect2 Recommended Reading List for the beginning of your marital education.
The TV and movies you mention where the kids are soaking up the conversations between ex-s and blended families, are usually complicated and maybe torturous.
Your point about serial relationships is well noted. We tend to end up picking another partner that has many of the same traits the last one(s) did it's often our inner work to heal the wounds so we have healthier relationships.