Mental performance needs practice just like physical performance. At some point you max out. And if you don't keep up practicing at level, your performance declines. Knowledge too is bound by those rules. We're constantly forgetting as we learn new stuff.

> I'll never forget that knowledge exists and where to find it if I should need to relearn.

Meaning you'd have to practice to again reach your former performance. Sure you may reach your peak level again and reach it quicker thanks to your old practice. But you do need to practice again. And then again, if you want to keep it.