The gjallerhorn will still be the best PvE exotic. It will still beat truth, dragon's breath, and the lmgs. So no, no one will say "well, truth is now as good as the gjallerhorn, I guess we don't HAVE to have a ghorn." There still won't be a replacement for the ghorn, it just does less damage. So to meet the same damage output, the fireteam will need more ghorns. It currently takes two rockets to bring down crota on hard. With the nerf it will most likely take three. So they just added another member of the team who needs a ghorn. So really, they just made the problem worse.