Lifespan of components?

I suspect typically 35 to 50 years or more , based on my experience… not much. But we all know capacitor performance falls after some time… probably less than twenty years. Then there is better performance in new stuff. Typically even in amps ten years is enough to get a very important improvement… say compare same models. For forty years I had Pass amps of equivalent power and design… ten years is very important sound quality improvements.

some components seem to never fail, but simply sound a little worse and then more worse, but still work...

Heat is major cause of component wear, manage heat and get longer life.

And then we have much faster evolution and innovation in certain audio categories. Take streaming for example, I consider these components consumables, certain components may be obsolete in matter of months.

Think about how many audio components are considered failures prior to parts failure. Many components may still operate yet would be considered failures as no market exists for resale.  I bet most of us have some equipment sitting around unused and not really salable.

1965 Macintosh MR71. 

Still have a couple original Mac labeled tubes. Over the years, a couple choice caps in areas were switched out. Just  has upgraded output jacks and original tuning cable replaced and indicator bulbs replaced. It gets  daily use, sometimes I've neglected to turn it off and find it's been on for days. 

The plan is to keep it going for another 40, when I turn 100. The original Mac tubes will probably outlive me.