Or... you can just use the top up built cell to manufacture things. Why build your own nanorobots when there are pre made nanorobots that already works.

Because switching to a different chemical composition (not carbon based) is more likely that way.

I think it must be possible to create equvivalent functionality from more durable materials.

This certainly is scify, but imagine changing all your cells one by one to cells which can withstand wider temperature ranges, and corrode (rot?) half as easily.