To be fair this sounds less like a problem with squishy prison abolitionists (as USDAs are appointed at the federal level and not elected locally) than a problem with the general incentive systems of DAs, which are to bring low cost, high certainty prosecutions. It's a complicated issue to be sure, one that is exacerbated by the fact that several state legislatures, at least one circuit court, and the supreme court continue to reduce the number of activities that qualify as "gun crimes" in the first place. The Fifth Circuit for instance, following the Supreme Court's Bruen decision, ruled that a federal law banning gun possession by individuals subject to a domestic violence protection order was unconstitutional. The idea that "lawful gun owners" aren't the problem is also problematic, because it ignores the fact that "lawful gun owners" perpetrated a lot of the most high profile recent mass shootings (Las Vegas, Buffalo, Uvalde, Nashville, Louisville).
Interestingly enough, I think throwing the book at people who commit gun crimes is an issue on which conservatives and liberals can be brought together.