If I'm doing some kind of outdoor activity, then I'm likely already carrying batteries for other things. I don't think it's the huge barrier most people here think it is. I guess if you're into ultra-light then yeah maybe it isn't the target market for you - but then why would you be carrying a huge watch in the first place rather than something light?
I mean, I tend to agree. FWIW, most people who do UL Backpacking use a smartphone for maps and a UL battery. I've taken my Apple Watch backpacking with me before, and keeping it charged wasn't a thing. Just one extra (short) cable to bring, I'd put it on the battery during dinner and camp setup.
The thing is though, I've only found it really useful if I'm doing stiff ascents and am not comfortable with my training. Mostly just to keep an eye on my heartrate, if my training/preparations are adequate for my trip, I don't use it at all.
Garmin Fenix cannot replace a phone, but has better battery life. The Apple Watch Ultra can do most of the things you'd need your iPhone for out on a trip.
If you are really UL, you can ditch the phone and just use the Ultra. Apple Watch + small battery is lighter than a Fenix + iPhone + battery.