1. BYU doesn't regularly fill LES now. Why add more empty seats?
2. You do realize that BYU has the largest stadium in the B12 already, right?
3. Outside of the B1G and SEC schools, BYU is already comparable to every other P4 stadium in size.
4. Cost to add additional stadium seating is simply not worth it. Don't get caught up in thinking bigger is better.
One other thing to think about ... are you sure BYU would even be interested in being a part of some semi-pro type college breakoff?