OOMS I agree with your assessment that if people "settle" for someone with whom they aren't in love, but the "life timing" is right, there will be inevitable problems further down the line.
And yes, I agree that old flames are especially intoxicating, for the reasons you put forward, but also because the person reminds us of our young self, when responsibilities were fewer or non-existent.
Often though I think this is less about the person - and more about the memories they evoke.
I suppose I am always surprised when mature adults under-estimate the potential effects of rekindling a thwarted love affair, invested as they have been with lots of possibly false beliefs about the "one that got away". It is in fact extremely likely that the emotions felt will be intense - and the effect is like a bomb detonating. I think it is also more "black and white" than you suggest, because it involves a series of choices. People say "yes" to a lot of steps before actually embarking on an affair.
I think another problem is the notion of "soul mates" or that for each person, there is their one. Realistically, it is extremely implausible that of the thousands of potential partners there are in the world, people happen to have found that person sitting next to them at school, or at work. This is something my H and I (gently) disagree on. He believes I am his "one", whereas I think we have evolved over nearly three decades to become deeply entwined...
I can think of exes where the physical attraction was huge, but as a fairly rational person, I know they would have been disastrous as a life partner, but conversely I would never have settled for anyone who didn't set fire to my body as well as my mind.
Those are the aspects that keep a long marriage alive - and the challenge is to grow together and keep that mutual fascination intact. I don't share your pessimism about whether that is achievable though - it certainly is, but I often think it comes back to that initial choice - and whether the union was based on a deep emotional and physical connection. If was there in the first place, it can be rekindled - and it helps the marriage sustain an enormous amount of life knocks and inevitable "troughs".
If you've "settled" though, you've still got life choices, but again the challenge is to make those adult choices with the least amount of pain to yourself and others.