Jay, some of the jobs are difficult even with a decent home workshop. I can't cut internal keyways, because I don't have a press-type keyway cutting kit (let alone a slotter), and keyways in internal tapers are especially difficult. Making chain sprockets - one of the most wear-prone parts - is also so difficult I'd never bother. I have a rotary table, so I can cut the teeth, but if you want it to last more than a year or two you need to use high carbon steel and heat treat it properly after machining, and I don't have facilities for that.
I agree with your point generally, but in practice I think you need to have access to a middle-sized toolroom rather than just a decent workshop. In a town or even a regional center, it doesn't seem practical.
