I usually prefer vulcans over machine guns, apart from SO builds that can actually get within machine gun range. They're much better at stopping missiles, a task which I've always found machine guns to be underwhelming for, as it's not uncommon for them to miss most of their shots if the missile or a fast fighter does not approach it directly in a straight line.
Flaks aren't underwhelming at all in their PD role due to their AoE, efficiency and reliability, and if you want a high firerate just use a dual flak or vulcans. Not to mention that converting LAGs into PD costs extra OP and also makes other weapons target missiles, which might not always be what you want them to do.
Locusts already shred hull, giving it any damage type other than frag would make it too powerful against shields and armor, and nerfing it's damage down to 25% of it's nominal value would make it too bad at anything but destroying light fighters.
The
is an absolute beast.
Amusingly, you forgot to mention the only gun with frag damage that is actually worthless: the Thumper which is bad at everything and can't even deal significant damage to the bare hull of anything but the lightest armor due to it's low damage per shot.
As far as I'm concerned, frag damage has it's place in PD weapons which primarily fight low/no armor targets, while keeping them from melting regular ships with ease (imagine vulcan cannons with energy instead of frag damage).