Replace the boots if the axle doesn't click when you turn and/or its hasn't been exposed to the elements for too long. The axle shaft wont go out on you as long as there is grease in the boot and no foreign objects are present to wear down any of the internal components.

Over time, yes the axle shaft can wear down to the point of needing replacement. But I would estimate that to be at least 200k miles. Nothing compares to the OEM axle shaft. Aftermarket ones may fit, but you'll deal with little quirks that comes with the reduced cost of one. Its just my opinion.