Because it came so late that by the time it came it change the way how the perk work in general which us I'll say KF1 true veterans couldn't get behind idea that you have no ammo to kill trash and crowd control because husk gun is limited in shots. In a way it's like demo got m32 that does 1/4 of damage... you're just not used to it.
That's why you want to redo the weapon. Give it extra range, extra stumble, extra AOE to make it a better direct CC weapon. It gives variation to flamer and it has a pushback effect. IMHO whole firebug needs to be redone. Compared to KF1 if you removed the "boomy perk" he would feel crap.
Claiming that KF1 true veterans didn't get behind it seems incredibly generalized. I would consider myself and a handfull of people I know to have many hundreds of hours on it, so it doesn't really speak for us. Maybe just the really, really vocal few. It is like on KF2, I know quite a few people in game who are incredibly good at it, but do not bother with forum at all, so their opinions are essentially unheard. The same was true of Killing Floor 1.
I personally saw it as a very good weapon, but people got very stuck on the idea that Firebug should only be about crowd control. I think that is too restrictive, and something like the Husk gun (that people were begging for for god knows how long) really changed it up a bit. I don't know what people were expecting the implementation would be when they were asking for it.
I would think that adding future weapons to Firebug could meet the role that you were explaining, I still really don't see the need to remove its Scrake potential. It would essentially leave one perk that essentially counters it. As for the skill tree of Firebug, I think the zed shrapnel skill should be moved to Demo, no matter how much I love it. But other than that, the tree is pretty decent. I would go as far to say that Firebug is one of the better balanced perks right now (excluding zed shrapnel).