Well at the very least it shows that regardless of how "good" the relationship is, there is a base level of compatibility. I've blogged about relationships in the past and how I think a successful relationship probably incorporates the notions of being healthy, happy, and productive weaved together with support and commitment.
This isn't really a blog post about if relationships are worth it (an entry for another time... spoilers: yes they are). This is more about how we can actually measure a successful relationship.
There are a lot of variables at play here: Is the person "better" because they are in a relationship? Is it because of their partner specifically? Does the length of time somehow make this more valid?
My inclination is that there must be some correlation between time together and success. That's because, as humans, we have self-preservation tendencies. Meaning if you've been with someone long enough it's generally because isn't bad for you. There are cases where relationships are toxic but they stay together or a couple stays together out of laziness/complacency - but I think those are exceptions. In general, people stay in relationships because there is value.
So my answer is Yes. It is a good measure of success but it needs to be combined with some other factors. Overall outlook on life and the ability to be pushed out of your comfort zone are important to a relationship. I blogged in the past about how going out of your comfort zone can lead to success and I think relationships function in a similar way. Individual growth is important no matter what. When in a relationship, growing together is equally important.
I know this entry isn't as satisfying as my normal analytical breakdown of things. "A successful relationship is measured by 1, 2, & 3 and here is why." That's because I don't truly know the answers. I don't think anyone knows discretely and precisely what constitutes "success" when it comes to relationships.