Linux has changed. Originally inspired by Unix, there were certain well understood but not well enforced rules that everyone understood. Programs did small things and used pipes to communicate. X Windows servers didn’t always run on your local machine. Nothing in
/usr contributed to booting up the system.
These days, we have systemd controlling everything. If you run Chrome on one display, it is locked to that display and it really wants that to be the local video card. And moving
/usr to another partition will easily prevent you from booting up, unless you take precautions. I moved
/usr and I lived to tell about it. If you ever need to do it, you’ll want to hear my story.
A lot of people are critical of systemd — including me — but really it isn’t systemd’s fault. It is the loss of these principles as we get more programmers and many of them are influenced by other systems where things work differently. I’m not just ranting, though. I recently had an experience that brought all this to mind and, along the way, I learned a few things about the modern state of the boot process. The story starts with a friend giving me an Intel Compute Stick. But the problems I had were not specific to that hardware, but rather how modern Linux distributions manage their start-up process.