Not a can't fill it problem. I really wish BYU students majoring in French literature would be required to take some business courses, same with those people running operations.
But look at the math. Chairs would actually reduce BYU's capacity by perhaps 10,000 seats.

That's 10,000 fewer gate receipts which at BYU is meaningful.
That's a much tighter limit on student sales.\
That's a loss of game day concession revenue of on average about $32/seat or $320,000.

In light of the last one, I suppose those of you putting thumbs up are all in on sharing the lost revenue by contributing to BYU Athletics.
Now in fairness, BYU has always drawn well when it was winning. The dwindling stadium crowds to 85% of capacity are related to a bad product.
In marketing events, production managers know that you do whatever is necessary to fill out the stands because the people will buy the concessions. So set the cheap seats cheap enough that the stadium is filled. See the concessions.

Now I get that YOU want to be comfortable and so getting rid of the Bench Seats is desired. Well good for you. Go root for Stanford. BYU's stadium keeps the tradition that still seems fine for the Rose Bowl, the Big House, Penn State, Rocky Top, Notre Dame, and most SEC venues. Benches are fine for the die-hard fans of those programs. Let's not be all "alpaca and silk" about the game like the wine and cheese eaters in Napa and for Cal. Let's be BYU and cram us in, be loud, be blue and be one of the most beautiful sites on a Friday or Saturday evening in all of college football. If you want a chair seat. Stay home!