It seems to me that the feelings of being "in love" come and go even when you're with the right person. You can love someone deeply and they can be the right partner for you even while your feelings fluctuate. If you leave someone as soon as the romantic passion fades, you don't give time for those feelings to eventually come back.
It does take work to sustain the passion and bring back those feelings of being in love. But it is definitely possible. I agree that it's not good to stay in a relationship if you're truly unhappy, but feelings change over time and they aren't always the most reliable barometer of how strong or compatible a relationship is.
I am with you that there might be more than one good match out there for every person. And sometimes two people can change and become too different over time. But barring some major incompatibility, there's a lot to be said for continuing to work on your relationship with the partner you've got instead of going out in search of someone else.