https://en.wikipedia.org/wiki/Complex_number#History
P≠NP is not like that.
If you view mathematics as the exploration of progressively stranger objects, that there's such a gap between discovering two roots should make us cautious of discounting things after a 100 year search.
We don't know what we don't know.
It was something like 200 years between the discovery of sqrt(-1) and more-or-less universal acceptance of -1 as a number in the first place (yes, imaginary numbers predate the universal acceptance of negative numbers). European mathematics in particular favored geometric interpretation of numbers over algebraic interpretations until relatively recently, and while irrational numbers come about very easily in geometry, negative numbers don't have an immediately obvious interpretation, particularly if you exclude it is a convention of directionality.