Excellent post. Personally, if I get 5 years out of a bike, I'm happy , considering the the rapid rate of development . Of course, I'm like a lot of bike riders who upgrades all components.
Curious phrase. Where are you seeing this "rapid rate of development" in ebikes? And over what period of time? Is it in battery capacity? 500 watt hours is still the industry standard among major manufacturers like shimano, bosch and yamaha, and has been for years. Shimano has introduced a new model with lower powered motor, as well as decreasing noise incrementally in it's existing top of the line motor. Bosch motors are a little smaller.
Juiced has introduced 52v batteries but they are unreliable, and none of their electronics can handle exposure to rain.
DTC frame designs are ridiculously outdated and offer zero design input from the brand owners who do nothing more than order from a catalog and specify color and decals.
Every DTC bike uses standard shimano or sram rear derailleurs and cassettes and chains. Some ebike specific chains are supposed to be a little sturdier. No test results yet to confirm this.
The only "significant" upgrade (perhaps not so significant) are frame integrated batteries. However, Stromer has offered frame integrated batteries for years. This design has been more widely adopted, but are far from universally available. It's nicer looking but doesn't offer any performance advantage.
With 99% of ebikes capped at 48volts, ebikes aren't getting any faster. With 500 watt hour capacity, they can't travel any farther.
I see solid, noticeable refinement of existing designs for the most part, rather than rapid, game changing innovation.
What evidence do you have that ebikes should last 5 years or less? Frames should last indefinitely, as should wheels. Batteries have to be replaced of course, but not the entire bike. Tires, chains, cassettes and chainrings should be replaced but they are wear parts. Bearings should be checked every so often but should last a very long time assuming proper adjustment and adequate grease. Saddles, grips and pedals don't wear out. Neither do handlebars or stems.