Chances are there may be extra 6 months or 1 year lifespan
extensions for the MK4s when January 2015 comes...
Higher chance some of the DM3500s (not yet extended to max 19
years) would get the extension compared to the Striders (if any
extension before their alr current max 19 years
lifespan).
If you're using the VO3xs as a cross-comparison, i would say
back then when they chose which VO3xs to extend before 19 years,
those VO3xs that extended had a reason to stay because the West
depots were lacking DDs back then.
But I don't really think HG or BN/AM (to a lesser extent, SLBP)
really lacks SDs currently in terms of SPs. They've been hoarding
the Citaros as SPs for a reason. 
