I think there are two issues really.
One is that, while most units come with prf weapons, commons don’t. They’re left in the dust a lot of times, and an inheritable DC weapon does free up the A-slot as suggested. IS wants DC to retain its value as an inheritable skill, and isn’t likely to make its access prolific. Despite additional effects, they still clearly value built-in DC. Say’ri was only half a year ago and got slapped with a negative condition for having a DC slaying weapon. They still clearly think DC is a big deal.
Second is the issue of lightning breath. While you could argue the value of any DC weapon just needing lower might and slowed cooldown to be balanced, I’d argue that’s not the issue. The issue is dragons aren’t good in the meta anymore. Lightning Breath would still be scary if dragons weren’t so universally countered. In their heyday, that freed A-slot allowed for one of the most powerful skill combinations the game could muster, thanks to Breath skills. It was so strong that most dragons would give up their prfs just for that combo. And now those are outdated with even stronger options. It’s not that Lightning Breath got nerfed. It’s that dragons got nerfed, and as a result, their DC weapon doesn’t strike the same level of fear.