"The opposite concept would be "one application, one system", which is pretty much a non-starter for larger companies, though it used to be PC dogma (Pournelle's law)."
No, "one application, one system" was never PC dogma. Most people didn't know how to make a PC multitask, but it was far from impossible. For example, device drivers were loaded into memory and worked alongside command.com. Early viruses used the same mechanism. Basically, if you could print from Word Perfect or VisiCalc, you were multitasking. I'd go into details on the TSR mechanism, but you're probably already yawning.
The actual quote was "one user, one CPU", in Jerry's Chaos Manor column in Byte Magazine in (probably) 1984ish ... I sold my complete USAian collection to an idiot on Ebay, or I'd look up the exact issue (the shipping costs alone were astronomical ... I'm not sure who was the biggest twit, me for keeping 'em for so long, or the dude who bought nearly a ton of obsolete, unrecycleable clay-coated paper and had it shipped from Palo Alto to Philly ...).
Jerry later updated it to "one user, multiple CPUs" ... He was a very early adopter of multitasking and networking of home computers. Seems to me that he also had something of a "law" that said something about checking the cables first ... But one of my early mentors was constantly harping about "check the wire first!" ... maybe the memories have merged in the mists of time.