In order to properly answer this question, you have to first consider the process(es) put into place to allow this.
It would seem evident that it's simply much easier and more efficient to moisturize the teeth with the lips, and retain that moisture. Considering that teeth are a fundamental part of ones health. The energy cycle would need to implement this change in dynamic if other energy was being used to reconstruct, or to protect the teeth.
In the instance of the lips, the saliva is a somewhat double-medium and the lips offer multiple functions.
So, without the lips - you actually have more issues as well that the lips help simplify. Such as disease, breathing in of air from a polluted climate, and simply not being able to really control what goes inside of your mouth.
So really, while this is the problem you're trying to solve - this problem is relative within the nature of the true solution that lips provide.
In short, it may be simplest to simply remove the teeth altogether and grind your food in a morter in order to consume it.
If you could somehow integrate iron production within the body, and produce a form of titanium/steel - that would be the ultimate solution probably.
But really, this question is much bigger than simply caring for the teeth - because the lips offer many health benefits and evolution always finds the best fit.