Why is it “Guten” but not the genetive form “Gutes”?
“Gutes” without a definite article would be uncountable:
Das ist zu viel Gutes. - That’s too much good.
“des Guten” limits it to a certain amount of good or one single good thing:
Das ist zu viel des Guten. - That’s too much of a good thing.
Both work, but have a slightly different meaning.
However, the set phrase uses the definite article.
Probably because it is normally about one specific good thing that turns into something bad if you overdo it.
Thanks a lot for your help!