Wow. That's crazy. Here in Ontario Canada we see lots of vehicles rusted out in less than 10 years old. Usually people's reasoning for not undercoating was they forgot or didn't have time or just they new every 5-6 years.I've lived in New England and NY my entire life (49) and never have used any form of undercoating. I usually purchase new and keep a vehicle 10-15 years and 200-250k. I have always gotten car washes with under spray soon after salt exposure. I haven't had a car/truck rust out on me yet. My old 2004 F150 is still going strong after 17 years. I do see certain suspension parts rust out eventually.
I'm not averse to under spray treatment, just never have. Might start late this fall.