I suspect it wouldn't be hard to find a case that disproves your assertion, if there's someone out there who tried polyamory for a while, discovered he/she didn't like it, and then discovered that monogamy worked better for him/her.

I know at least one person who never identified as a poly per se but was in an open relationship for a long time. Now she is in a monogamous relationship. I assume it is because it works better for her.

Also: chickens?