I agree with LivTokyo that this is a very personal decision. I think that we all have to weigh our priorities differently based on what is most important to us - none of us can 'have it all'. I also think we all have a different fate when it comes to our relationships, no two are the same. In some ways you are very lucky and many women would envy your situation having a good husband and father to your child. However, you'd also find many women who would believe that without that passion, the relationship is doomed and it isn't fair to anyone.
The passion tapering off is a different scenario than it never being there in the first place. I empathize deeply with you because I can relate to your dilemma, but I've only been with my husband for 4 years. Things happened very quickly for us because I got pregnant after 3 months of meeting him. We were totally in love, but had to weather all kinds of stressful and very rocky times, the kinds of things that one deals with when you tie yourself to someone that you don't know yet very well. And there are many other issues too, but at the end of the day, we love each other very much and always have.
The problem is that since we had our son 3 years ago, our sex life has really suffered. Sometimes I think it is me that is the problem - hormones and all. Sometimes I think it is because I'm so in love with my son that it makes it hard to have that same passion for him too. Sometimes I think it is because of all the fighting and conflicts we had during the first few years, that has really affected my desire. At certain points my heart just closed him off, out of self-protection. Sometimes I think it is because of all the stress and responsibilities I had to take on, much like you said about your own situation, I always feel like I'm responsible for everything and that at times he is like another child I need to care for.
But then, there are other times when I look back at my life before him, the relationships I had before him, and make some very telling observations. I've had more passion in other relationships, and I never had sexual issues before. In fact, if anyone, I'd be the one complaining that it wasn't enough for me! How ironic. But! Many of those relationships were dysfunctional and the men were not was I was looking for in a partner, certainly not men I wanted to have a baby with. When I met my husband, I knew right away that he would be my husband, and I wanted to have a child with him. I was no spring chicken either, so my eagerness definitely contributed to how we "accidentally" got pregnant so fast.
I think we've done the best we could in the situation we had to deal with. We both experience such great joy in our son that there have never been any regrets on either side. I find him physically very attractive and always have, but after the baby I was in a lot of pain for months and had a hard time getting back into it. Meanwhile, he was like a salivating dog wanting to pounce me at any chance he had. This started an unhealthy dynamic that we've never really broken out of. I just needed a break, a little time to heal and to pay full attention to my baby (not to mention I was really overwhelmed by all the new responsibilities of parenting, exhausted, we were broke, arguing, etc.) and he would never give me that space, or let it be okay. Instead it was a constant guilt trip, or chasing me, begging, feeling so rejected and laying it on me. It was very hard. But I think some part of me lost respect for him then.
Now he's better about it, but our sex life is still okay at best. And I'm sure he'd say it is lousy. Sometimes I feel really sorry for him and like he deserves a woman who can satisfy him more. Sometimes I also think I'd be better suited with someone else. But I am also not willing to let go of the love, the care, and the security we have a family... not now anyway. As for stability, we have never had much but after the new year we had a huge blow out and I was ready to leave him, and since the we are working things out little by little. It's the first time I feel hopeful in a long time. I don't know, if that "spark" doesn't ever return, I doubt that we will make it in the long run. For now though, the time we share with our son as a family is so important to me. If there is anyway we can make it work I would like to. If I was 28 and not 38, I'd probably have moved on by now. Whenever I think of us splitting I feel profoundly sad. I pray that one day if our life circumstances change and the things we are working so hard on and sacrificing now will pay off and allow for more time together (we have none), more fun, more bed room time when we are not both totally wiped out. Much of the time I have this sense that we really are meant to be together, problems and all. Other times I worry that if we don't regain the level of passion, eventually one of us will stray away.
I think that you can compare your own story to others but in the end, it is a path you need to navigate on your own. Best of luck to you and your family!
EDITED: Check out "Committed" by Elizabeth Gilbert if you're interested in learning about different concepts and expectations of marriage, worldwide.