It sounds like you've really thought this through, and you have good sense. Keep thinking carefully about these sorts of things.
I don't think people are replaceable. Every relationship I've had has been unique, and has had its own dynamic and impact on my life. I've never replaced anyone.
I do think, though, that it's reasonable to understand that most relationships end. If you're married, you have about a 50/50 chance of making it through to death, regardless of whether you try to stay monogamous.
I actually think that polyamory can help a relationship continue, as it can adapt and grow as people grow. It also takes a little pressure off. You don't have to be all and everything to your spouse, and he doesn't have to do that for you. If you can break free from the "ownership" type of thinking, then you can probably adjust to this, too.