This is a straw man and a false dichotomy. There’s a huge middle ground between flawless science and total bullshit. Plenty of room in there to draw a reasonable line.
Sending someone to death row because some guy wrote a book full of nonsense he made up about blood spatter (which has literally happened) is well over that line.
Climate change denialists make the same arguments for doing nothing because making things better won’t make them perfect.
If the only way we have to convict someone is nonsense about bullet markings matching barrels (and to be clear, it is nonsense. Not just low accuracy, but total made up nonsense) then we shouldn’t convict that person. That’s the whole point. If your evidence is all garbage, the right thing to do is let the person go, not convict them just because you can’t find better evidence.
