I'm interested to know what forum members consider to be a good lifetime for an e-bike, and particularly for an e-bike kit motor. How many miles for a given price would you consider acceptable?

This question was suggested to me by a YouTuber who was expressing some dissatisfaction with his CYC Photon motor. He said something like "For this price ($950) I would expect to be able to go 10,000km without serious problems."

But even that - $1000, let's say for 10,000km is still 10 cents per kilometer, or 16 cents per mile. For a 50km ride (30 miles), that works out at $5. Is that a reasonable cost - given that on top of that there will be battery degradation over time?

As a ballpark figure, my first e-bike motor was a very basic hub motor from a Chinese company which cost about $400 including the battery, and it has already done 9000km and seems as good as new (though the initial battery died).

Compare that with my Tongsheng TSDZ2, about $400 without battery, which is causing me multiple problems at about 3,500km.

Back to the original question: what is a reasonable expectation for an e-bike?
In 2018, I paid $350 for a 1300 w geared hub motor, 20 A controller, pedal pickup & magnet wheel, switched brake handles, throttle. The plastic gears wore out in 2 1/2 years or ~4500 miles. The controller lasted 5 years before a hard "Error 8" that was inside the controller, not the sensor. I would say that is fair.
In 2019 I paid $700 for a 1000 w Mac12t motor, 40 A ASI controller, throttle. 18 months later a rainstorm shorted & burned the pins of the ASI controller, burning up the harness on the motor too. I was disappointed about that. Pins were too close together to withstand 48 v. It took me 2 years to figure out how to get the motor apart so I could replace the harness from the circuit board out. I used rectangular white hall effect pickup connector and .157 bullet connectors for the phases, not the weird tiny pin connector Luna shipped. Luna products are suitable for the Mojave desert racers, not for anywhere it rains. Now the motor is working fine again with a 25 A controller bought off ebay. I really like the high torque Mac12t motor for hauling 60 lb groceries & ag supplies up hills of Clark County.
Between the ebikeling & the Mac, I bought 4 used 350 w bafang geared hubs taken from the scrapped Jump fleet for $36 each. They were not suitable for hauling groceries up hills. I burned the winding of one and another developed some other problem, both in under 500 miles. Fine with one light rider around flat Washington DC I guess.
I was just thinking about the cost of owning and riding an ebike and if it really was an economical choice.. If I pay $2000.00 dollars for a bike and I have 2000 miles on it so far it has cost me one dollar a mile. To get the cost down to a quarter of a dollar a mile I would have to get 8000 miles on it. Of course you have to figure in the cost of maintaining on top o the initial cost.
Cyc motors are somewhat infamous for being persnickety, with annoying issues that should have been dealt with during beta testing, except the company uses its retail customers as the beta population. I have an X1 Pro v1.0 so I lived this dream and it has persisted with the later introduction of the stealth, and now the Photon. Cyc gets away with it by being generous with replacement parts.

But... thats only the tip of that iceberg. I'll leave it there.

I am guessing you are talking about Cap Codswallop's comments on that motor in his High Voltage Kits channel. He's much more than a Youtuber, he's a highly experienced member of the DIY ebike motor community, with a wide breadth of experience across many different motors.

The life of the motor - in particular a mid drive - is greatly influenced by how the rider rides

and at a deeper level, how well the DIY builder followed the largely-unspoken-all-in-one-place rules of the game with regard to the initial build

Only when you get those two areas sorted can you start looking to blame the equipment.

With all of that said, My BBSHDs are at their oldest, in fine shape dating back to 2017. Mileage on them is in the thousands of miles. About 4000 for the one that gets the most use, which was built in 2021. And that bike still has its original chain on at 4000 miles so there's where the whole 'ride it right' thing starts coming into the picture, after building it right. But since I have a half dozen bikes my mileage is spread across all of them over the years so I can't point to one and say its been good for 10000 miles.
I would think a gearless direct drive motor would last forever, but for me a hundred miles riding one might feel like forever, because of the weight and lowered ability to coast. I think the nylon gears on the geared motors are pretty durable if one doesn't encounter severe conditions,

I have over 5000 miles each on a pair of tiny geared motors that my wife and I ride. . Generally left in assist level 1, where they only pull 100 watts, Downshift and pedal harder on the hills. I think a high cadence is best for biking, with or without motor,

Most of my ebikes were conversions, and I have lost a few controllers to water damage before I got better at keeping them dry. I did drown the controller on a store bought ebike too. Have broken a half dozen mirrors and probabky the same number of LCD displays from the bikes falling over.

Batteries of good quality can last a while. All three of the batteries I bought in 2015 are still in use, but I have seen cheap stuff fail quickly.