Fair point and my mistake. While I am comfortable changing pads, I must acknowledge that some people aren't comfortable enough, or simply don't want/care to work on their own vehicles. I can see this being around $1,000 USD at a dealer if you wanted to pay full price for pads, sensors and labor. So, if you're worried about costs and don't think you'll reap the full benefits, then maybe the rear locker should be avoided. FWIW, I drove my 2016 L494 HARD (I used to run 1/8 mile drags with it, I Auto-x it once, took it out to a local race track for some light laps during open lapping sessions and yes, took it off road!) and my rear pads lasted twice as long as what you're describing.
Fortunately, since this is an order/option discussion thread, I feel like it's OK to stray a bit here from the purely "Defender" nature of the conversation to discuss our experience with this option on similar LR platforms. All things considered, the rear torque vectoring diff on the Defender is an option that I would check if I were building one. 95% of the users on this forum would likely do the same, but I guess the beauty of a thread like this is that we can get opinions and weigh the pros and cons