In particular, it’s a crucial point whether physicalism has to declare by fiat that anything that exists or happens has a lawlike physical basis, thus in effect relabeling what most of us would call “non-physical” in ordinary circumstances. Leontiskos mentioned Nagel’s The Last Word, and as usual Nagel puts it well: “I [want to] interpret the concept of ‛physics’ restrictively enough so that the laws of physics by themselves will not explain the presence of . . . thinking beings in the space of natural possibilities. Of course, if ‛physics’ just means the most fundamental scientific theory about everything, then it will include any such laws if they exist.” If that’s all physicalism amounts to, then you’re right, it adds nothing conceptually. — J
I think this highlights an important tension within physicalism. Broad physicalist interpretations certainly add nothing conceptually, but the more physicalism is restricted, the more objections arise. There's an ironic tradeoff there where in order to make physicalism meaningful, you pretty much have to make it wrong or at least so problematic as to be questionably worth defending.
This feeds into Hempel's dilemma as mentioned by
@Michael "if physicalism is defined via reference to contemporary physics, then it is false — after all, who thinks that contemporary physics is complete? — but if physicalism is defined via reference to a future or ideal physics, then it is trivial — after all, who can predict what a future physics contains?"
https://plato.stanford.edu/entries/physicalism/#HempDile
Hempel's dilemma wasn't exactly what I had in mind writing the OP. The later criticism I discovered by Chomsky though is almost identical to my main point as he essentially makes the same argument--that movement beyond the boundary of methodological naturalism adds unnecessary and undesriable metaphysical commitments, i.e. creates the empty suitcase.
Here are the clearest formulations I've been able to find online so far:
https://plato.stanford.edu/archIves/sum2020/entries/physicalism/
"[Chomsky argues that] the physicalist project in philosophy of mind is on the face of it rather different from the naturalistic project. In the first place,
the physicalist project is, as we have noted, usually thought of a piece of metaphysics. But there is nothing metaphysical about the [methodological] naturalistic project, it simply raises questions about what we can hope to explain.
...
It is precisely at the place where the physicalist project departs from the naturalistic project that Chomsky's criticism begins to take shape. For insofar as it is different from the naturalistic project, there are a number of ways in which the physicalist project is questionable. First, it is hard to see what the project might be — it is true that throughout the history of philosophy and science one encounters suggestions that one might find out about the world in ways that are distinct from the ones used in the sciences, but these suggestions have always been rather obscure. Second,
it is hard see how this sort of project could recommend itself to physicalists themselves — such a project seems to be a departure from methodological naturalism .but most physicalists endorse methodological naturalism as a matter of fact"
And:
https://teorievedy.flu.cas.cz/index.php/tv/article/download/271/293
'
Chomsky’s objection is that the doctrine has no clear content. He thinks that those who advocate physicalism and those who would endeavour to prove it false are both wrong in thinking that there is a substantive doctrine at issue. Chomsky holds this general view about physicalism because he also holds the more particular view that the concept of the “physical” (or the “material”), which must inevitably enter into any characterisation of physicalism, is devoid of clear meaning.
At best, talk of “the physical” acts as a placeholder for whatever we discover, or could discover, to be true about nature. He writes:
"There is no longer any definite conception of body. Rather,
the material world is whatever we discover it to be, with whatever properties it must be assumed to have for the purposes of explanatory theory." '