It's a statement about what reality would be, after doing some counterfactual surgery on it. I don't see why that disqualifies it from being used as a baseline. I'm not entirely sure why it does qualify as a baseline, except that intuitively it seems obvious. If your intuitions disagree, I'll accept that.

It does intuitively feel like a baseline, as is appropriate for the special place taken by inaction in human decision-making. But I don't see what singles out this particular concept from the set of all other counterfactuals you could've considered, in the context of a formal decision-making problem. This doubt applies to both the concepts of "inaction" and of "baseline".

This isn't the case, for example, in Shapley Value.

That's not a choice with "all else equal". A better outcome, all else equal, is trivially a case of a better outcome.