There is a big difference between being a quite junior captain on the 88, with zero movement the past five years, versus being (supposedly) a lot more senior on the 717, with a rapidly increasing category size over the next couple of years.

Many of us don't have an obsession with wearing five black stripes on our shoulders, especially if by doing so we are junior and staying junior for years on end.

However, the upgrade to captain (complete with higher pay) is a lot more appealing if you are either not junior to begin with (which is what a lot of senior 7ER F/Os are anticipating) or will remain junior for a fairly brief time as your category gets larger and larger about the same time as retirements finally kick in when that process is done, thereby keeping the upward movement continuing.