I am assuming your term of stunning is captive bolt. When thinking of having farmers do this to every single pig, I don't think it would be that easy.
Lets take a 1,200 head barn to put some numbers to this. First you would need someone to snare all 1,200 pigs. You would most likely want more than one person to snare because it is not that easy to do and is very tiring. Second you would want more than one captive bolt gun to have one person loading the charges and one person doing the shooting. Third you would have to have someone shoot all 1,200 pigs. Personally I would not want to do that. Pulling the trigger that many times will have a far greater detrimental affect on a persons mental health than almost any other form of depopulation. In packing plants they have automated their method of stunning, electrocution, so the workers don't have to have a hand on approach to that process. The workers are there to make sure the machine is working and doing it's job.
Personally, I would not call it animal abuse. Yes, it is not perfect and there most likely is some suffering of the animals, I would still not call it abuse. Refer back to what one of my professors said.
Most if not all plants are back to being open, but at a reduced capacity. Where I'm working at we send pigs to three different plants normally. When covid started to hit, we had loads being cancelled due the the plants trying to shuffle the pigs around to still try to meet everyone contracts as best as they could. For a few weeks we were selling/shipping half as many loads as we normally would. We were trying to find other places all over the country to send pigs to, somehow we found places is Oklahoma and Arkansas and a few other states that aren't very close to where we are located. Most other producers were doing the same things. We also were trying to slow down the growth of the pigs, between diets and different ventilation set points in the barns. We slowed down a lot of pigs. Projections when plants were being hit the hardest was we were going to have to depop 100,000 market hogs, luckily we only had to do one barn of 1,200.
Now take your method of stunning every animal and apply that to 100,000 pigs. It's just not practical of feasible. And we are just one company, and there are hundreds more companies out there all facing this same problem.