Our current cumin masters (neodymium and sarin) had their warranty expired in January 2016 so they're pretty close to our usual five years lifespan. Since they also need to be updated to stretch (T177385), it might make sense to now get replacement hardware, install it with stretch and decom the old systems.
The current servers have 24 cores and 32 GB of RAM (which were mostly unused), the replacements can even be smaller systems.
(The Cumin masters are no good candidates for Ganeti IMO (e.g. to ensure that we still have Cumin even if Ganeti is down)).