There is no history of US top tier mainline carriers flying aircraft for 40 years. It’s never happened. Northwest Airlines flew their DC-9’s to 35 years and probably ended up regretting it. I can’t even imagine opening up a 40 year old 767 for an invasive D Check. As far as comparing a 35 year old mainline 767 that flies around the clock 24/7 with a C5, N641UA has over 130,000 hrs on her, your average USAF C5 flys about 80 hrs a month. Guy/gals please do your research before posting on here, UAL is absolutely ready to replace every single 767 and 777-200 with a 787, they just cant get them fast enough…I get what you guy want to happen, I lived 14 years of my life on the 75/76, still my favorite, but time marches on, sadly.