So here's my opinion on the question from 25 years of experience with DIYing BMW brake systems.
You can leave the rotors and just change the pads at 71K. However you will find that the new pads will last longer than the original rotors. This is usually the case because OE BMW rotors are somewhat "soft" and wear while in service and usually do not last through two sets of pads and end up with warped rotors and 35% pad life left (estimates).
Additionally, an ideal brake "relining" includes removal of the rotors and resurfacing them on a brake lathe. The resurfacing removes even more rotor material and makes them more susceptible to warpage from heat as the rotors continue to wear. All that said, it is just better to replace the rotors and pads as a set since some of the cost of the brake job should be for resurfacing the rotors, which can be offset by just buying new rotors instead. Meaning, if it costs $20 to resurface one rotor and a new rotor costs $65, the $20 is better spend just getting a new rotor.