Fact is you throw the watch away due to the battery being dead more so than the tech being outdated. It's bad design to make a device with limited functionality such as a watch with a non-replaceable battery.
You're being a revisionist here. Originally your argument was that the watch was totally useless after three years and might as well go in the trash because the battery couldn't be replaced at all. Now you're saying that battery is too difficult and costly to replace, and there value of the watch is too little after three years. You've adopted the points that I've made without acknowledging it, which makes it seem like that was your position all along.It still is an impractical value over time for the devices limited use to not have a non friendly replaceable battery. In mint condition the stainless steel watch is worth $50 to $75 used. $80 to replace a dead/dying battery costs more than its value.
Let me interject this argument before I continue to my main point. Most watches that cost more than $100 don't have user replaceable batteries. And even those that do, I wouldn't call the process "friendly." In fact, I would challenge you to show me ANY watch whose battery replacement process is "friendly." I don't consider dealing with tiny screws that get lost the instant you unscrew them "friendly." But I digress. My Tag Heuer watch, for example, requires that I either send it to Seattle to have the battery replaced, or I take it to a Tag Heuer certified jeweler. And it typically costs $100-$120. When taken in that context, the process of replacing the battery in the Apple Watch is typical.
Now, addressing your NEW position. Yes, it probably is not worthwhile to replace the battery in the original Apple Watch at this point. But that device was under powered when it was new. I think three years from now, when people are contemplating replacing the battery in their series 3 vs. buying a new series 6 the calculus will be different. But that is just a guess on my part, and I could easily be wrong. Nobody should buy a SS watch today assuming they'll replace the battery in three years and continue to use it, just like nobody really does that with cell phones. BUT, I would bet dollars to doughnuts that three years from now any series 3 still in good working order will be worth more than nothing and throwing them in the trash will be foolish. You know, just like it is today with the original Apple Watch.
Finally, I reject your original premise that the residual value should play any role in the decision process anyway. I dare you to show me a Timex that was $20-$50 new which is worth the cost of a replacement battery on the used market.