I work on mowers that range from 40 years old to last years models and I woul not dream of upgrading .
Every year mowers come out with more "desireabe" features and less durability in order to be $ 50 cheaper than their competitor.
So every time you upgrage you will need to be looking at a higher, thus more expensive replacement.
Add to that the down time working out which blades work best for your customers, getting used to the new mowers size & controls etc etc etc.