The whole line stops if a single process has problems.
Given a 99% chance of working for every single process for hundreds of processes you end up with... well... not much output.
Given a 99% chance of working for every single process for hundreds of processes you end up with... well... not much output.
When I was younger I had a less severe but equally enlightening experience. A transmission case line with 24 CNC stations had our UI software running on PCs to make them easier to use. They could run fine without the fancy UI but it was how the operators had come to run things. I went in one afternoon to load a software update from floppy disk. The plant manager came out. "What are you doing?" "I'm updating this UI software." "You work for $company?" "Yes." "Is there gonna be a $company guy on site for 3rd shift tonight if something goes wrong with that software?" "No." "You aren't gonna update it then." I went home.
Go fast and break things doesn't work in that environment, and yes the critical processes work far more than 99% of the time because as you say there wouldn't be much output. It sounds like that's the exact lesson Tesla is learning now.
[edit] the reason all the processes on the line have to work is because it's basically impossible to store a bunch of cars (or doors or engines) anywhere until things get fixed. Ad-hoc fixes are just not feasible at that scale.