Just so everybody is aware, this dichotomy between modal collapse and ungrounded reality is not forced by the setup. There is a third option.
That option is the free choice of the necessary agent. Multiple choices (worlds) would be compatible with the necessary being. This being has a will. Agent causation therefore adequately explains the choice of the agent without that choice being a necessary part of its nature. In other words, the choice is contingent, because it's free.
The next move you often see here is the demand for a contrastive explanation, If A explains B, then why B rather than C? If both are compatible with A, what distinguishes between the two?
There are multiple reasons to doubt that this is a cogent request.
1. “Rather than C” does not automatically introduce another fact requiring another explanation. From the fact that A does not rule out C, it does not follow that A fails to explain B. It follows only that A does not provide the particular contrastive explanation being requested. The contrastivist owes us an account of why every adequate explanation must discriminate between the actual outcome and every relevant alternative. The PSR promises an explanation of contingent facts, it does not obviously promise an answer to every contextually specified contrastive question.
2. Requiring a something prior to discriminate B from C may beg the question against libertarian agency. Suppose we invoke a Pruss style defense where God freely chose B for certain reasons, thereby explaining B without necessitating it. The contrastivist introduces some further fact D and asks whether D explains why God chose B rather than C.
> If D necessitates B, then the choice no longer appears libertarian.
> If D just favors B, the contrastivist can still ask why God acted on that consideration rather than choosing C.
The explanatory demand either continues or eventually terminates in something that determines the outcome.
The Free Willyin answer is that the agent, not an additional antecedent event, is what settles the question. God’s reasons make the action intelligible, while God’s exercise of agency makes B obtain.
Ontological grounding mirrors epistemic justification: chains either regress, loop, or end with an ungrounded brute fact. A necessary being doesn't escape this. Its free choice to create this cosmos is either necessary (modal collapse) or ungrounded--a brute fact by another name.