There's a kind of "conservation of expected evidence" here: if the critiques had succeeded, you'd have reduced the probability of AI risk, so their failure must push you in the opposite direction.

This doesn't sound quite right. I think what matters is the amount of cogent, non-redundant thought about the subject demonstrated by the failed critiques. If the critique is just generally bad, it shouldn't have an impact either way (I could randomly computer-generate millions of incoherent critiques of creationism, but that wouldn't affect my credence)

Similarly if a hundred experts make the same flawed counterargument, there would be rapidly diminishing returns after the first few.