Oh, I agree. I know she's expressed a slight sense of... envy, perhaps? at the relationship my partner and I have, but I've tried telling her that the only reason we both have this relationship (and are as connected as we are) is A) because we were willing to do the thing she is unwilling to go (that is, let go of long term relationships that were not healthy for us nor connected, choosing ourselves and peace of mind over longevity of marriage) and B) because we have been through the tough stuff, and learned that we are each okay on our own without a partner, tend to value our partnership and make great efforts to each let the other know on a regular daily basis how appreciative we are.) I really think it is not either / or, but both / and. If I had tried to make things with my ex work just because we'd been together for a decade and a half and I kept hoping things would go back to the early love bombing days, I would not be here now.

FWIW I see a lot of your well-phrased sentiment in a lot of people, and notably my brother-in-law: "the measure of time is only an excuse for not moving on." He's been with his wife since they were 15 and all they do is fight and nag and bicker and tear each other down. He has told my partner that he's jealous of our relationship...but hes also unwilling to choose himself over the longevity of marriage.