A large cottage industry has evolved in the selling of old Leica glass which would have you believe that its the best thing since sliced bread. Monetary or intellectual interest in having one believe that this old glass is better than anything new is used to make subjective pronouncements about old glass. The real truth is that the glass is in some cases as good as some of the newer glass but in most cases its not as good, its just cheaper and one can feel comfortable in the assertion,
"While more recent Leica (and Zeiss) lenses are arguably "better" than the older ones (very arguably - many people prefer the signatures of the older ones), that does not at all make the older lenses obsolete or unusable.
For example, many people prefer the older 35mm Summicron version IV to the current Asph. one."
Of course the other assertion is that in current manufacturing
"Materials used are often cheapened compared to the past!"
Of course the last absolutely useless thought, only of use to those that sit and think about lenses at thier keyboards is
"That also brings up the question of lens generations: The Leica 50mm M Summicron probably has at least five generations (too lazy too look it up now) - each with different optical schemes and signatures and each with its admirers and detractors."
I have an M3 with a set of RF lenses of that generation and in my direct side by side comparison using K64 under a 5x Leica magnifier on my Normlicht light table, I can't tell the difference between the 'old' RF and the 'new' ASPH glass. The build is the same except that the ASPH lenses are lighter because of the plastic barrels instead of chrome, often substantially lighter.
As I stated "some approaching thier lifetime which was not known when developed. " Leica/Leitz had no idea of the lifetime of the materials being used at the time of manufacture, so your question "Pray tell what is the "usable lifetime" of a lens?" is non-sensical.
I will always purchase the newest lens I can afford rather than using semantical arguements about qualitative lens factors that can't quantified.
I have some theories about why some lenses degrade and under what conditions but they are just theories and I don't have the time to let a set of old lenses age under different conditions.
I have no desire nor could I care less to discuss different lenses, I just purchase good equipment and use it.
What is Posted was,
"I would go with new glass other then Leica if i could not afford new or relatively new, say last 15 years or so Leica glass. Bad glass is bad glass no matter whose name is on it." and my Post still stands.--Dick