There's a balance when you're dealing with a partnership/long term relationship. Yes, absolutely, there's an expectation of independence and individual stress management. Just like there's an expectation, for most people, that their partner contributes to household chores, bringing in income, etc. Everyone has to pull their weight in every relationship, lest one person feel overwhelmed or burdened.
On the other hand, being in a long term relationship also means that you're a team. If one person is incredibly overwhelmed, it's understandable that they'd lean on their partner. That's your person. Who else would you lean on?
If one person is leaning too much in an otherwise healthy relationship, I don't think the answer is to throw in the towel. There have been TONS of times in our relationship where both my husband and I have been guilty of that. It's a really natural thing that can happen. We talked about it, we made a plan, and we moved on. Those plans have included the leaner (and sometimes the person being leaned on) getting some much needed counseling. It's included reaching out to other friends, family members, and colleagues. It's included seeking out stress management tools to make our lives run efficiently and help us minimize stress. There are a lot of options if you and your wife are willing to work on it.