This debate comes up maybe every 5 years. In the long run, it just comes down to warranty voiding. Car makers argument is only based on the loss of $$ from people not purchasing extended warranties at time of purchase. Your average car purchaser trades a car in every 4-5 years as to where your typical tuner doesn't purchase the ext/warr because they understand that as soon as they tune/tinker, it is gone anyways.
Too many aftermarket manufacturers out there that stand against it. Plus, MANY aftermarket parts actually are BETTER for safety and economy, so the green delegates love to sing to that tune. My old STi was "quite" tuned/modified. After all was said and done, it was pushing more hp/tq, ran cleaner, and even got better mpg. Be damned if I will ever stop tinkering.
Gary Noonan
'01 S/C VX / '18 Forester XT