Your comments suggest it's not cost neutral. There is no guarantee that the operating costs, which rise over time, equal the residual value of the car when sold, which declines over time. My guess is that you lose $ compared to not having it, which means it's lifestyle creep on it's own.
I think it's legit to consider whether the benefits are worth the cost, but it's still lifestyle creep.
Alternate choices: Offer to sell the car for mom & dad, dedicate the money to Fun With Grandma and Grandpa Fund that pays for outings where the kids play with your parents. Or, say that you have enough cars, but you would be willing to buy stock for the kids with the proceeds. Or just offer to solve the "what to do with car" problem by selling it for them and letting them keep the money - be a good son, show you have enough money to say no, help them out. All these options convey strength and Mustache ethics. Not saying you have to do these, just that they're options.
As is accepting the lifestyle creep. You're a person, not a Mustache robot. Your call. Deep down, which option will you feel is best when you look back on it ten years from now?