I understand the "No one is irreplaceable" way of thinking. If someone leaves a hole in your life, the hole will be filled. Whether it's filled by another person, a hobby, a renewed focus on what you have, etc. It does not remain empty.
We walk around on eggshells when we fear losing people, but the reality is that no particular individual is a necessary ingredient for our happiness. In reference to love and friendship alike, that would be what I mean if I say, "No one is irreplaceable."
It sounds cold and hard, but if you find yourself in an unhappy or unhealthy relationship, the words have value.
If you're in a healthy relationship, a happy one, one that progresses through difficulties, then you really don't need to sweat the thought.
I would encourage your partner to do some "hard work" himself. You know what they say about an ounce of prevention? It's true.