The batteries last a long time, they don't just give up unless you forget to charge them (and we're only talking about a couple of hours).

This would be like someone running out of fuel in a car. A known limitation which is completely avoidable.

The good thing with my current bike is that it never runs out of fuel, by design. I can pick it up anytime and it just works, it was so 10 years ago when I acquired it, and it'll be so 10 years down the road.

I have a Garmin Forerunner 735XT watch and it needs charging every 15 days, which is better than any "true" smartwatch out there. At some point in the distant-but-not-so-much future the battery will flat out die and there will be no replacement part. The experience from my mechanical watch is entirely different: it keeps on ticking as I move around, and the thing will probably end up surviving me.

Going from zero to non-zero - whatever the value of non-zero is - is non-negligible mental weight. Zero is a freeing experience.

Although with 20 years of use, those gear cables are likely to fail. Presumably you do things like lube the chain, inspect your tyres, keep them inflated to the correct level?

I've had a couple of Di2 batteries fail. One of them went suddenly and I had to grab a train to get home. The other one lost capacity quite quickly, so I'd have to charge it once a week until it got so bad I needed to charge it fully each day until I swapped in a new one.

So when that happens only the back or front runs out of battery. Usually it will be the back and you just swap them and ride home without being able the shift on the front.

Back and front are powered by the same battery which typically lives in the seat post. When the battery gets too low, the front shifts are disabled first as they take more juice.

Not on mine, SRAM etap, there is a small rechargeable battery in the front and back. They are the same so can be swapped.

I really, really like it. I know I paid way too much for that bike but it is amazing and I can really feel the difference in a way I never could before.

Why aren’t these bikes using a hub generator to produce the necessary energy to automatically recharge the batteries for the bike computer and shifting motors? Or am I missing something?

Di2 is mostly popular among the performance cycling set who likes to go fast. The ~5% drivetrain power loss you'd get from a dynamo isn't attractive in that market. Also, Di2 requires charging every couple of months. It just isn't worth it.

I have seen some randonneuring bikes with a dynamo with USB charging for topping up the bike computer and the like. Commuting and utility bikes tend to just use the dynamo for lights.

That’s a market that’s entirely foreign to me, I use my bicycle entirely for commuting, buying groceries, etc so I’ve got a hub dynamo powering the lights.

I’m honestly surprised that there’s no super-low-power hub dynamo for this segment, but if it’s all people using bicycles for planned trips, they can take charging into account when planning trips.