A big issue is that UPS had/has a younger pilot group than other airlines like the "Big 3" legacies and FedEx. As such, the percentage of the group hitting mandatory retirement age each year has been minuscule compared to those four companies.
That is going to change in the coming years, however, as half the pilot group reaches 65 by the end of 2028. Unless the airline shrinks precipitously (not counting 6 and 1354, has it shrunk any since the 8 went away?), attrition alone will allow for movement not seen here in the better part of a decade. Of course growth would be nice, but we all have expectation zero about that.
People here like to point to 2 year upgrades at Delta along with "know a guy who knows a guy" that made $350k+ as a narrowbody captain due to Green Slips and profit sharing, without regard to the reasons why NYC 88A/717A goes so junior or how the GS process works (or the time/work investment required to take advantage of it).
Past results are not indicative of future performance, so I believe it unlikely that the stagnation and long upgrade times faced by UPS 05-07 hires due to parking 3-crew planes, Age 65 and global recession will impact 2014-on hires in the same fashion.
Again, flying for UPS is a good job (contrary to what some here might think) but it is just that - a job - just like United, Delta, American, Southwest and FedEx are.