80 point bucket range.

I have a question, if we're going to make a system that results in senior pilots not flying on reserve while pilots a few % points below them, or even 1 number below them, pick up all of their flying why don't we make a system where the senior regular pilots get paid alv but don't work and their flying is done by a pilot a few % points below them for no pay? If its fair for reserve why not regular line holders?