You can't assume that ASCII will be more readable than any other binary format.
Yes we can, the combination of simplicitly and ubiquity mean it is highly unlikely we will lose the ability to read it.
UTF-8 is a little more complex but the encoding method can still be described in less than a page, the harder bit is what to do with the code point sequence you get from decoding but for most widely used languages* that is a simple table lookup.
Do you have any tools that can open and read PETSCII?
Well you might end up with swapped case and block-drawing would be a mess but you could read the actual text by just treating the file as ASCII.
* The exceptions being languages like hebrew, arabic and some indian languages.