Boeing is not in the business of keeping older aircraft flying or enhancing the abilities of older aircraft. Boeing is in the NEW aircraft business. Due to the fact that the Boeing sales unit controls the strings around corporate, all of commercial aviation has suffered. Our customers have suffered as well in higher ticket pricess and environmental distruction of our world.
Boeing has countless fixes for issues found troublesome to the operators and have done little to nothing unless the help enhances Boeing's ability to sell new aircraft(ie. Boeing gives the fix to an operator to help them find money to sign up).
Boeing also has countless TSFC reduction packages that are not nor will they be ever offered to the commercial public. As early as 1985 Boeing knew how to reduce the 747-200 SFC burn in cruise by 8% but they didn't want to hurt 747-400 sales by breathing life into the -200s so they killed the program.
Look at the hushkit market that Boeing so nicely stayed out of so that they could sell lots of 757s and 767s..... Boeing thought that without their help the 727s and classic 737s would die.... Nope, Boeing threw 5 billion dollars out the door to the STC market and still haven't sold crap for 757s or 767s.
Boeing would love to put an absolute death date on each airplane but thats just not how it works. Only the FAA can do that.