There’s a fallacy presented when someone makes the argument that more of A on some continuum is bad(good) but rejects the idea that less of A is good(bad) thus reducing to either the current status is necessarily the best or worst of all possible statuses. I can’t remember what this is called.
If a fallacy is an error of argument, this is not necessarily an error. Consider the continuum of states of affairs. Suppose this continuum covers all the possible, or at least realisable, states of affairs in a particular situation.
s1, s2, s3, s4, s5, s6, s7, s8, s9, s9, s10
where each state after s1 represents more of a certain good. Why shouldn't it be the case that s1 - s5 represent a deficiency of that good, and s7 - s10 an excess ? In which case s5 is a kind of Aristotelian 'mean' (mesotes) or proper balance between excess and deficiency. (Aristotle, Nicomachean Ethics, II.7). On this reading s6 is the best of all possible states of affairs.
Can't see any fallacy here. You may have a different kind of example in mind but I'm working on the data given.