As a pilot the problem with upgrading early is seniority dictates QOL. A junior captain needs to work a junior base, fly a junior plane, and get whatever crappy trips are left in the schedule. That means that a senior First Officer would have to move or commute to a new city until they build enough seniority to go back to where they want to live all while working the worst schedule, or even worse sitting on reserve. Absolutely not worth the pay bump when you can wait a little longer and not have to deal with all that.