Waiting periods do appear to have a small but significant effect on reducing gun homicides according to the research I’ve seen, although I haven’t looked at data specifically for Maine, which is a state with a lot more guns but a lot less gun violence than the average.
(The study I looked at appears to show that background checks increase gun homicides so I don’t trust it very much, but a Rand meta-analysis also claims that that small but significant effect is real.)
My point isn’t that waiting periods are bad policy but rather that they’re an irrational response to this mass shooting. (And it is this particular mass shooting that convinced Maine to pass the law.) Gun violence that would not have been prevented by a waiting period is evidence against the efficacy of waiting periods, but here people are responding to that evidence by increasing their support for waiting periods. It’s contrary to basic logic.