And you can run a differential for that long with the same fluid too. And most vehicles will be just fine with that. I't's not a made up thing to change it, most makers do have a recommended interval for such. Like completely swapping out brake fluid, it's a thing most people never think of, and for how long they own it, it wont matter.
Certainly wont harm it to change it out, not like power flushing an older transmission. How much does he want for this service? My guess is he was a unit designed for this, and it makes him mad profit margin. I would have to look it up, but doing this at home shouldn't be crazy hard. Easier than trans fluid, but not as easy as a differential.
Overall, not a thing I would sweat. I'll be changing the Chargers fluid out, but that's becuase it's big heavy wheels and wide tires stress the hell out of it's system, like the tie rods.