Not to be overly facetious, but do you have a testability-ometer? If not, then I think your claim that ideas are untestable is outside the realm of science and into the realm of philosophy. Of course I agree with you that the brain-in-a-jar stuff - as a truth-claim - is definitely untestable, but it appears you are saying that that brain-in-a-jar stuff is intellectually empty and the idea that it is intellectually empty is intellectually empty.
The practical problem with that from a science perspective is that what appears untestable at one time might turn out to be testable at another. Sometimes people have to come up with new idea that we can’t test in order to be inspired to figure out how to test them (or to inspire someone in the future to figure out how to test them).
I’d argue that another problem is that untestable ideas aren’t “intellectually empty”. They can instead be totally functional in a scientifically verifiable way. Functional to talk about if not functional to actually test the truth value of. A discussion about being brains in jars might really discussion about anxiety about a person’s place in the world.
Whether or not we have a purpose is, to my mind, probably a scientific question that we don’t yet have the apparatus to confront (someone else may argue it’s just a non-scientific question). But whether or not a belief that we have a purpose has an effect on mental health is well studied - it does.
Anxiety is real. We can see it (or something highly correlated to it) on FMRI scans. (Aside: this is a good example of something that would have been thought of as untestable a hundred or two hundred years previous - not untestable as in “we can’t test it yet” but untestable as in “it’s utterly inconceivable that this could ever be tested; if this could ever be tested my entire idea of what reality is would crash down around me”). So now that “intellectually empty” is dismissing people addressing real health issues. Because processing their emotions through metaphor works. Therapists use metaphors to help patients talk about feelings all the time, and they developed that technique by trial and error, by notes, by experiment, etc., that is, through science.
Which, to go back to the topic of the thread, is I think why people get annoyed with what they perceive as “scientism”. The issue is that someone can take the idea that we can only have knowledge of testable hypotheses, apply it, and end up saying that someone taking physically measurable actions (such as communicating to a friend through compressions in the air) that have a physically measurable effect on the functioning of their brain (e.g. whether their auditory processing works properly or produces hallucinations) and say that nothing is happening there, that it’s empty, that it is outside of science.
It’s not that science dismisses those things or diminishes them. It’s that some people cloak their dismissal of those things in science.