you think that all things could not fail to exist/obtain?

No, but that might be the case.

but the argument you've been given above isn't speaking of physical necessity, but broad logical necessity

Logical possibility tells us very little about what is actually possible. Consider that there's a possible world in which a person knows there is no God. There's no contradiction in this idea. There's also a possible world in which a person knows there is a God. Both are logically possible. But one of them necessarily is not actually possible, because God is either exists or doesn't exist by necessity. This is why logical possibility doesn't tell us enough. Logical possibility is a necessary condition for actual possibility, but it isn't a sufficient condition.

So the fact that one can conceive of things not existing, or existing differently, without logical contradiction, does not entail that they are in fact contingent.