LOL....I like your car metaphor. Economic professors have proven if you walk for 5 years, the money and stress you save from not having the clunker at all, allows you to pay cash for a new one. Isn't it better to have no debt.
You appear to be rationalizing your situation.. The problem is, I see you said that you do not think you can do better than this; that he is the best you have so far. My issue with this is that it point to a low expectation of yourself and others.
Clearly he is not available for marriage to you.
There are many instances where couples who failed as lovers and married partners, continued as business partners and general friends (no privileges, no booty calls).
The trouble is, it sounds like you and he are fitting together in a way where the rocks in your head, fill the holes in his; and his rocks fit your holes....BUT, what you really need is two people who cause each other to be the best they can be.
In some circles, for example, the Judith Wright Institute of Exceptional LIving in Chicago, believe it takes 80 or more dating relationships of short duration, to find that one in a million person who will truly compliment you for the rest of your life.
I recommend taking a look at Judith Wrights web site, where you will find an email for her. Send her a note and ask if she is still having women's empowerment weekends. i recommend them. http://www.judithwright.com/groups.php
I believe you can do better for yourself. Simply because you are both imperfect is no reason to continue to live like that.