They nailed things down in the 20's-60's with one methodology. Then the world changed and the machines changed.
It's not just the method of construction that has changed. In the 1950s, mechanical devices were cheaper than electrical devices. Today, in almost all cases, the opposite is true.
I think that helps to give the newer machines a reputation of being complicated and unreliable. Unlike mechanical devices, electronics don't gradually wear. They typically work fine until the day they completely die.
If a spur gear in a 70 year old machine wears out or breaks, we just measure it up and cut a new one, or find someone with an old drawing and cut a new one to spec. But, if you have a machine full vintage electronic gizmos, your options are limited. Often you are looking at some kind of complete system retrofit.
Some of the worst machines to keep running are those transitional era machines from the 1970s. For example, I worked on some American CNC lathes from that era with large spindle bores. These were built as CNC lathes, but they are really just converted engine lathes. They have something like 18 gear ranges in the spindle and no constant surface speed. Even worse, the servo motors had a gear reduction for the ball screws. So the servo goes through a pair of helical gears to a ball screw. The screw had a resolver on it for position feedback.
Anyway, they wanted to do a control retrofit, but it was going to be hugely expensive because the servo drive gears were worn and the backlash was going to play hell with the servo tuning. So the decision was to eliminate them which required fabing up complex adapters.
I've seen the same thing on Devliegs, especially the J model machines that were built as CNC machines. They were CNC machines, but they were really just old manual machines that they added CNC to. So they have all the complexities of the manual machines plus tons of unreliable vintage electronics and controls.