Originally posted by monochrome My best performing lenses in terms of solid feel, durability and accuracy are my Takumars. They are approximately 40 years old, and I see no reason they should ever stop working as long as they are periodically serviced - say once every 20 years.
Same for my mechanical cameras, as long as there are parts (or donor bodies) and film available.
Sorry but talking about legacy lenses is a different context altogether to a modern AF lens, which is frankly quite a lot different construction wise. For one thing, lenses today have to factor in AF. Lens designers have to contend with the size and weight of the focusing ring and focus throw as it has a bearing on the load and inertia the focusing motor has to overcome.
If one were to open up an SDM lens, you will be very surprised to find that the SDM motor is about the size of a button or small coin. My take is some of the SDM lens failures we hear about is probably because early production SDM lenses had not so robust motors. Talking with my local Pentax agent, those lenses that had these SDM issues were sent back to Japan and had their motors replaced with new ones with no subsequent issues arising. One of the early adopter pitfalls.... heck I just recently saw with my own eyes a Sony user's new Sony A900 had its mirror lock up rendering the camera useless... apparently a known issue.
One thing I know is the DA* lenses are pretty solid in build quality, which I can't really say for the DA and some DA Limiteds, the common loose and wobbly lens barrels come to mind.