for various reasons my hack/training bike has just been ridden and ridden (pretty much daily) on a diet of minimal maintenance for about a year. It was old before then. I normally reckon to keep it in a condition whereby I would happily ride 50- 100 miles on it at the drop of a hat. Recently I have not been sure if there is any single part that would last another 50 miles or not.
I could make a list of the faults but it would be long and boring. Suffice it to say that I have recently removed the remains of the front mudguard (35 years old, broken into three pieces) and fitted some new brake blocks (they were worn to within 0.5mm of the holders before....
This only leaves about a day's work to replace the other worn out parts and resurrect the gears from their present 'three working' state back to their usual 'ten working' condition. That would still leave a bike where most of the major parts are 35 years old or more, and have done at least 65000 miles...
BTW a test of whether your bike is truly knackered or not is, if you took it to bits, whether you would bother building a bike up from the pieces laid in front of you. In the case of my hack bike there are only about three bits that would get a 'yes' vote....
cheers