You mean the original MBA with a spinning hard drive or an SSD MBA? If you mean the newer ones, sure, yeah, if you rarely write to the internal drive, it will live a lot longer than if you do not. But, under normal use, the lifetime is probably going to exceed the usefulness of the product (unless it is subjected to harsh or unusual use), so the tradeoff of using an external solely to extend the life of the SSD is probably not worth it.