I've been around since well before the first home computers were built in the 1970s. I actually built an 8080-based PC (of course I built a processor from SSI, too, but the 8080 was a bit faster ;-). I worked with two fussbudget original IBM PCs in 1982, one of which wouldn't boot until you dropped the keyboard from 3 feet off the table, then you had to find and snap back on the keys that went flying - this was NOT an "IBM quality machine" to say the least! I lusted after the Amiga, played with an Atari ST, enjoyed MacOS 1.0 and the much improved 2.0, and finally grudgingly adopted DOS and then Windows, and finally (oh the bliss!) discovered Linux.
Having lived through the entire personal computer revolution, I can say with the fullest confidence that you are talking utter and complete nonsense.
While the first paper describing the concept of a digital signature was presented in 1976, it was purely theoretical. The first commercially available digital signature system was introduced to the market in 1989, many years after Apple, Atari, Commodore, IBM, and a thousand midget start-ups created the home computer boom, the home computer bust, and the establishment of the de facto IBM Personal Computer standard.
Each and every one of these computers would boot anything you put on its front switches, paper tape, cassette tape, stringy floppy, 8" floppy, 5 1/4" floppy, 3.5" floppy, ZIP drive, optical media, or USB flash drive. (See, I don't need to do the research - I have my own memory of every one of them! Would you like to see my copy of CP/M on 8" floppy? Still got it. Binary code for a 256 byte football game I wrote? Still got it. But I digress...)
Since the ability to require digital signatures followed the IBM PC by about 7 years, I think we can be confident it hasn't "always" been the case that personal computers were limited to the vendor's "approved OS", nor is anyone demanding a "change" to keep systems open.
By the way, I wrote an operating system of my own for my beloved Atari 800, after reading "De Re Atari" which documented every bit of the interface. It was pretty primitive, but dude, it was NOT digitally signed! :-D