Anecdotally, as someone who was doing software development in the late 1990s, yes we were doing it wrong, and Agile spoke truth to that. Getting this message out was an important change. And if that took "a few consultants gathered at a ski resort" issuing a manifesto, then so be it.

Was "Everyone" doing it wrong? Mathematically improbable. Were the vast majority, the mainstream, stuck in a dysfunctional waterfall system? Oh hell yes.

And the usual answer to the failure of up-front planning was "Clearly we didn't plan up-front enough, we need plan even longer". It's a self-reinforcing pathology. So is the use of JIRA, but that's another story, post-agile, not pre-agile.

XP, Scrum and agile-adjacent Lean all pre-date the manifesto. The manifesto just tried to put all these ideas under one umbrella.