I'd argue that the tools (if they're 'good' in itself) reflect the principles with which they have been designed. And no, I don't think that these principles are eternal. We should look at the context under which these principles have been formulated or evolved, compare it to today's context, pick out the principles that still apply and throw the rest out. As an example, building small programs to do well defined, easily testable tasks and chain them together, is certainly still a good design principle. Strings as the universal interface, however, is not in my opinion, since it just doesn't reflect how most of today's UIs are built on objects - thus we have today's disconnect between the terminal and the GUI.
You believe that text as a universal interface isn't a good design principle because it doesn't reflect how other UIs, built upon completely different principles, behave? Is it not possible that it remains a good principle that WIMP GUIs simply do not adhere to? (Also related, "everything is a file.")
Unformatted text (as in text that doesn't follow a machine-understandable format like xml or json) is not a good interface today, I absolutely believe that. Imagine if "ls" would return an xml table that you could easily reuse in other programs, including graphical editors. Imagine that the OS could understand what every column represents and give it a name. You could do something like
ls | showtable in.modified in.filename --sort:1.
Of course you can do this today with sed, but in a very unintuitive way not easily accessible for beginners.
