One needs to see if the drop also occurred in states that did not have such gun friendly laws. Lott argued that he co-varied such out but that's been controversial.
You would need steeper slopes in the gun states vs. the nongun states to make that point. Probably been done but I don't recall the gun states being steeper. Also, you need to look at it by a county by county basis to control for demographic differences.
The point about there not being a noticeable increase in crime is well taken.