It's been done before, sorry.
It's never been done before, too risky.
It's too obvious.
It's too obscure.
It's too easy, everyone can do it.
It's too hard to launch, it'll never work.
Too indy, why can't you get backers?
Too mainstream, the man has polluted you, you sold out.
It's never been practiced, you'll do it wrong.
You've practiced it too much, it can't possibly be fresh.
Not here, this city/market/audience is too jaded.
Not here, this city/market/audience is untested.
The market has peaked, nothing goes up forever.
The market is dead, it'll never catch on…
Most bestsellers are surprise bestsellers, because there's no sure thing, at least not where we want to look for it.