Are you saying that in your games, a big bad has never been down to a few hp? He/she was always taken out while at high hp levels?
Taking out a big bad with a 3 damage chip attack can come up far more often and is far more likely than completely wearing down such a foe with such attacks. To me, a finishing shot with autodamage feels so meh.
The bad guy morgue will be filled with corpses all with the autopsy report reading: Cause of death-standing in proximity to great weapon.
Let's unpack this a bit more shall we?
What has to happen for DoaM to drop something?
1) The creature has to be reduced to 3 or less HP without killing it.
2) The GWF fighter's turn must be next in line, or, everyone between him and the last attack which reduced the creatures to 3 HP has to miss.
3) The GWF fighter has to miss. Which, by rights, he shouldn't be.
We're talking a pretty corner case here. At low levels, when creatures only have 3 HP, sure, it's pretty common. But, after 1st or 2nd level, it's becoming less and less often.
Which brings me back to a question I asked earlier that got lost in the scrum - is it DoaM that tips people over or killing something on a miss? If it's DoaM, sure, fine, that will come up every combat. If it's killing on a miss, then we're talking something that might come up once a session at the absolute outside and likely far less often than that.