Resistant Counterpossibles

A quick follow-up on Joe's kangaroo post (I couldn't resist). In his forthcoming book The Philosophy of Philosophy Timothy Williamson defends the view that all counterpossibles are true, including:

If Hillary Clinton had been my mother, she wouldn't have been my mother.

and

If the law of excluded middle had failed, 'p or not-p' would have been true.

Williamson then writes as follows (symbols have been translated into English):

If all counterpossible were false, 'possibly-A' would be equivalent to 'if A had been the case, A would have been the case', for the latter would still be true whenever A was possible; correspondingly, 'necessarily-A' would be equivalent to the dual 'not-(if not-A had been the case, then not-A would have been the case)' and one could carry out the programme of section 3 using the new equivalences.

As this is a counterpossible (if Williamson is right), what Williamson just said is true. But so is:

If all counterpossible were false, 'possibly-A' would NOT be equivalent to 'if A had been the case, A would have been the case', for the latter would still be FALSE whenever A was possible; correspondingly, 'necessarily-A' would NOT be equivalent to the dual 'not-(if not-A had been the case, then not-A would have been the case)' and [so] one could NOT carry out the programme of section 3 using the new equivalences.

I am not saying that counterpossibles are sometimes false. My only point is that the subjunctive mood is a common way in which to refute one's opponents in philosophy (perhaps the most common way), and so, if counterpossibles are true (per definition), philosophy isn't as deep and interesting as one would have thought.