If I was buying a Lexus, and planned to have it serviced only at the dealership, I’d consider this kind of prepaid deal, especially if I could confirm it saved me money.
I should note that, since I’m frugal, I have no interest in owning a luxury brand. Also, I’m no fan of dealership service departments. Tires, for example, are more expensive at a dealership, and if you buy them at a tire retailer, like Goodyear, Firestone, or Discount Tire, you can pay a little extra for lifetime balance and rotation service that costs much less than paying each time. (Discount tire might tell you they rotate and balance for free, but that just means they included the cost of lifetime balance and rotation in your initial purchase.)
So if you can afford to own a luxury car, and you can afford to drive and maintain a luxury car, I don’t see why you’d want to be frugal about car maintenance, but this isn’t a complex math problem. If you plan to have the car serviced exclusively by the dealership, and paying in advance saves you money, I don’t see any reason not to do it.
In the end, if you’re sweating over 15% of your car’s maintenance costs, a luxury car might not be the right car for you. Luxury cars are more expensive to buy, more expensive to insure, more expensive to maintain, and more expensive to repair, so owning one isn’t a good idea unless you’re the kind of person who has disposable income to spare.