I worked on big government projects using military standards that more or less imposed the dreaded 'waterfall'.
What was our mantra? "Design a little, code a little, test a little".
Agile re-invented or popularized various ideas that had been around for a long time. As always, you adjust for the problem at hand. Royce, the so-called inventor of waterfall (he presented it as an obviously flawed concept, a straw man), argued for copious documentation. Documentation makes a lot of sense when trying to built a jet-liner, and is often a time and money sink if you are trying to write, say, research code for computer vision.
So, "revolutionary"? I dunno? Certainly many places had taken the deliberately flawed Royce model as the way to do things, and so there was a big change as far as that goes. But I can't recall a time where some form of iterative/evolutionary development method was not known about and practiced, or where decision making was favored over process.
To make this clearer - the Royce-ers patted themselves on the back, and then got hopelessly entangled with process. Yourdon came along. Wow, was that a great process - which everyone got entangled with. Then there was Booch..... And now, we have Agile. The details vary, to be sure, but the fundamentals remain the same. Some good ideas for approaching engineering a solution gets turned into conferences, endless books and training, independent consultants, certifications, requirements to be certified to win contracts, and on and on.
I'm old.