The MD-11 is a great plane as it is. But as many other great planes its basic design dates well over 30 years back in time to a period when large engines were less powerful, ETOPS was unknown, FBW controls was only being tested on the Concorde and military planes, and composite structures were far less advanced.
So for "improving the MD-11" I would simply buy an A330 or B777 instead. That's also what the airline companies do these days.
But the MD-11s, which are already there, they will hopefully cary on for the next twenty years. As it looks today most of them will be cargo converted. They will of course last longer than those DC-10s which during these days are being converted to MD-10 at considerable costs.
The fact that such an extensive modification program is viable, and that 30+ years old DC-8s and DC-9s seem almost impossible to retire, it proves that Douglas (and MD) made high quality planes. Too bad that they couldn't make money on it. They failed to continue development of modern designs during the 70'es and 80'es, and it doomed them in the 90'es. Was is too tense competition, or was it too greedy shareholders? Probably a combination of both.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs