You're looking at it from the vantage point of "the future". For some span of time, "plastic never degrades". Then someone discovers something new, and after that plastic degradation becomes possible (though not widespread).
What we know changes.
What we know changes.
To call something a "forever chemical" is talking in absolutes, and we should know that most things aren't absolute.
This may be a bit pedantic, but: it's not that what we know changes, per se. We weren't mistaken in thinking that plastic doesn't degrade.
We mean: as of now, we have no possible method to make plastic degrade. It _may be_ we will develop such a method in the future, but it's reckless to rely on the _possibility_ of a future discovery.