5/8/2018 7:05 AM
We just replaced our 2007 Flex 21 with @3500 hours of use this year. During this time, we also had our older 2003 21's as emergency backups and for use after topdressing. During that ten year period I replaced the reels w/bearings twice on the 2007's. I could probably have whipped another 7 years out of these units before they needed some major overhaul of the transmission or in-depth bearing swap involving seized parts. People forget just how expensive a walking greens mower is to purchase new. They are not very involved to service. At worst, you might need an engine replacement or expensive transmission part to keep them running reliably for years.
So to answer your question about lifespan, (provided you have a system of backup mowers to facilitate having one go down for an unforeseen problem monthly) a really really long time.
We, and most other people, replace them when more features become available on newer models.