I guess we may be speaking of two different types of "old"... 
The mid-20th century mechanical lenses sold under countless brands (some of which were cheap and some not so much), and the obsolete AF lenses like, say, Canon 16-35/2.8 non-II or the vaunted 50/1.0 L. 
The first category I don't know a whole lot about, the second, well... lack of spare parts is a known risk, but otherwise, some of those lenses are pretty decent. I had a Canon 17-35/2.8 L for over two years, and apart from a very conservative MFD (1.5 times that of its modern successors) and lack of weather sealing, that unit from 1996 was surprisingly modern and well-heeled. As a rule, of course, these lenses had been updated for a good reason...
The mid-20th century mechanical lenses sold under countless brands (some of which were cheap and some not so much), and the obsolete AF lenses like, say, Canon 16-35/2.8 non-II or the vaunted 50/1.0 L. The first category I don't know a whole lot about, the second, well... lack of spare parts is a known risk, but otherwise, some of those lenses are pretty decent. I had a Canon 17-35/2.8 L for over two years, and apart from a very conservative MFD (1.5 times that of its modern successors) and lack of weather sealing, that unit from 1996 was surprisingly modern and well-heeled. As a rule, of course, these lenses had been updated for a good reason...