You're making an awful lot of assumptions. I suspect a lot more thought went into developing a standardized battery platform for these $10k+ mowers, given the average lifespan of a ZT mower, than your $100 portable drills. While I think it's reasonable to believe the OEM may not offer replacement batteries by the time the mower deck reaches end of life, as I stated previously, I do believe there will be aftermarket options available for many years after the OEM has moved on to newer tech.
Probably has more to do with usage pattern. Some have the free time and schedule flexibility to never have to mow damp grass, while some of us seem to be almost always mowing damp grass in early morning or well past sunset. Put a mower away wet 20x per year for 20 years, and it's not going to last anywhere near as long as the guy who only mows his lawn at 3pm on sunny days.