The essential problem I see is that a large number of Dems that vote in primaries are anti-gun; they are not interested in preventing violence and suicide, they want to ban guns. If Dems not only dropped gun control, but embraced 2A rights (while presumably attempting to address gun violence and suicide rates in more productive/effective ways), they would hemorrhage those anti-gun voters. At the same time, it's unlikely that they would be able to pick up pro-2A single-issue voters, because the Democratic party has such a long history of attempting to ban guns and enact strict gun control that those voters wouldn't believe it.

It would take multiple election cycles for Dems to break that belief, and you couldn't have any Dems breaking ranks in that time. All it would take is one Beto "dumbfuck" O'Rourke (seriously, who says they want to ban guns while running for governor in Texas, unless they're intentionally in the tank for Republicans?) to undo any progress they might make.

To be entirely clear, the people that are anti-gun will not accept any level of gun violence as being acceptable. It doesn't matter that the US has more guns per capita now than it ever has at any point in history, and that violent crime is at the lowest level in 40 years; it's all about the feels to them.