Even if you can hold a line upgrade isn’t always that great of a deal. Most 777/787 trips are commutable both ends and productive. A junior line holding 737/320 captain will have to fly more days, plus most likely give up days commuting the day before or after a trip. In my seniority I’d end up flying or commuting 5-6 extra days a month to upgrade for about $40K a year more. This gig is a package deal for me. I value days off pretty high. Some people are not wired for long haul flying. It’s nice to have a choice.