They could just not sell every single seat in the main cabin.
I’ll go back to my first point: The only thing more important that security to airline companies is “money”.
Your suggestion is that the airlines lose money on sale of seats because they’re keeping some seats empty on every flight on the off chance the airline (or one of their competitor airlines) needs to get a pilot or flight crew member to another airport. That would lose the airlines money. They wont’ do that.