Take X to be some view; take Y to be some simple and obvious objection to that view that is either based on internal coherence or facts that everybody accepts.
1. If X is accepted by many philosophers and/or taken seriously even by philosophers who reject X, then, Y should not be seen as a defeater for X.
2. The antecedent of (1) is true.
3. Therefore, the consequent of (1) is true.
On (1), the reason why is this. If X were open to obvious defeat, then it is doubtful that many philosophers would accept it and even less doubtful that philosophers who reject X would take it seriously (it is open to simple and obvious refutation after all!). Now, this does not mean that Y does not defeat X, only that it is most likely the case that Y defeats X only by defeating the defeater given for Y (or something even more intricate!). None of this, of course, entails that X isn’t open to obvious refutation by Y, only that we would be out of bound epistemically in thinking so (this last clause is intentionally vague as I think the idea as a whole can be communicated without needing to nitpick details). For those who think the premise is too strong as is, one can add a “probably” after “then” and achieve much of the same result.
On (2), no claim is actually being made here, but if some person would want to propose some position as X, then they would have to defend it.