If you say that ID is not science because can't be falsified because it can't be tested, and then you say that the ID claim that a biological feature is too complex to have arisen by Darwinian mechanisms has been falsified, you're undermining your claim that ID is not science because it can't be tested and falsified.
First, supporters of ID like the infamous Michael Behe seem not to want to take notice of evidence that falsifies a claim (such as how the supposedly "irreducibly complex" bacterial flagellum works with fewer components (and so is reducible), and is close in structure to a secretory system, so could clearly have evolved from it)
If you are tacitly admitting that the infamous Behe's claim of IC is falsifiable, then you can't at the same time say it's not science.
Second, as a empirical matter, to the best of my knowledge the function of propulsion via a rotary motor and a propeller in a bacterial flagellum has never been demonstrated to work with fewer components. Knock out experiments repeatedly produce the same results.
... is close in structure to a secretory system, so could clearly have evolved from it
The definition of IC has always referred to the function of the system as a whole, not to possible functions of it's individual components.
Saying that because a bacterial flagellum is close in structure to a secretory system, it could clearly have evolved from it, as if the mere logical possibility were tantamount to it actually having a homologous origin is a non sequiter which ignores the logical possibilities of convergence and coincidence, not to mention the logical possibility of common design. Such an extremely low threshold for scientific credulity, imho, evinces a presuppositional commitment to Darwinian mechanisms that is independent of, and not derived from scientific evidence, particulary with regard to the bacterial flagellum. A little healthy scientific skepticism is in order: Which came first, the secretory system, or the flagella?
Second, the claim that a structure is too complex to have arisen naturally is virtually impossible to prove. The number of evolutionary routes by which a structure could potentially arise are probably always very large indeed. Features can arise from simpler systems gaining complexity, or more complex systems becoming simpler, and this can involve systems which have quite different functions. An attempt to show that a structure had to have been designed has to involve eliminating the possibility of any evolutionary route to the structure. That is quite a challenge.
It sure is. Darwin knew the impossibility of proving a universal negative too, which is is why his subtle shifting of the burden of proof away from his own theory was so rhetorically clever: "If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."
The irony in your post is that you set out to demonstrate the weakness of ID theory in its resistance to falsification and what you inadvertently end up showing is that in principle there is no experimental evidence that could possibly be found that would falsify the contention that complex molecular machines evolved by Darwinian mechanisms.