There's a lot of truth to this, can't say there isn't.
My relationships tend to run around two years. I have one at ten years, and another (current) at four. I look at my SO now and feel comfortable in knowing that she's the best fit for me.
That being said, I've shopped around. I've met my hundred thousand people (part of the job) dated fifty or sixty of them, and had LTR's with seven of them. Out of all of those people, my current SO is the best, and I can actually run the math and say beyond a doubt that the chances of me meeting anyone 'better' is so infinitesimally small that it is waaaaay more likely that I will die before that happens.
When I see posts like the OP, where there has been an almost zero chance to explore opportunities after reaching a new stage of being (grown up from teen to adult) I look at that and say Well, duh. He's literally a different person now then when they started dating, and has no means of knowing whether or not she's actually the best person for him. There's no comparison, there's no data, just a feeling that is remarkably different than the way he felt before.
To me that makes his wanderlust an extremely rational thing. Even if he had the very best person life had to offer him, there would be no way to know if that was the case. It makes sense that a person in that situation would make some effort to figure out the truth of it. That struggle takes on a variety of different forms, and it's different in everyone. I think some people just 'choose to believe' that they've made the right choice and run with it. Sometimes doing that is the very thing that makes it the right choice after all.
In the end, people need to have their relationships and feelings validated by their experiences, and if they don't have those experiences, they will go out and find them- even at the cost of what they already have. I think I could successfully argue that it isn't even about making the 'right' choice, but resting secure in the belief (true or untrue) that you've made the best choice.
And that comes easier for some than others.