To answer the original question: YES!
Let's try an experiment: Let's replace the word "monster" with some other words and see if a similar sentiment makes any sense.
We already have plenty enough monsters. Why would anyone want more than what's already out there?
We already have plenty enough non-player characters. Why would anyone want more than what's already out there?
We already have plenty enough spells. Why would anyone want more than what's already out there?
We already have plenty enough magic items. Why would anyone want more than what's already out there?
We already have plenty enough campaign settings. Why would anyone want more than what's already out there?
We already have plenty enough adventure modules. Why would anyone want more than what's already out there?
We already have plenty enough fantasy fiction. Why would anyone want more than what's already out there?
Let's try an experiment: Let's replace the word "monster" with some other words and see if a similar sentiment makes any sense.
We already have plenty enough monsters. Why would anyone want more than what's already out there?
We already have plenty enough non-player characters. Why would anyone want more than what's already out there?
We already have plenty enough spells. Why would anyone want more than what's already out there?
We already have plenty enough magic items. Why would anyone want more than what's already out there?
We already have plenty enough campaign settings. Why would anyone want more than what's already out there?
We already have plenty enough adventure modules. Why would anyone want more than what's already out there?
We already have plenty enough fantasy fiction. Why would anyone want more than what's already out there?