It really is just plain text. Human writing systems were always this hard, and "for many years" what you had were separate independent understandings of what "plain text" means in different environments, which makes interoperability impossible. Unicode is mostly about having only one "plain text" rather than dozens.
It is not mandatory that your 80x25 terminal learn how to display Linear B, you can't read Linear B and you probably have no desire to learn how and no interest in any text written in it. But Unicode means your computer agrees with everybody else's computer that it's Linear B, and not a bunch of symbols for drawing Space Invaders, or the manufacturer's logo, if you fix a typo in a document I wrote that has some Linear B in it, your computer doesn't replace the Linear B with question marks, or erase the document, since it knows what that is even if you can't read it and it doesn't know how to display it.