I doubt "people into serious running" run into the 10s of millions that Apple are aiming the watch at though.
(I agree with your point though - I have a 910XT for the battery life - the Apple Watch would be useless as an activity tracker for me since the iPhone 5S doesn't really cope with 5h+ activities.)
I agree with you too. The Apple Watch is a mass market product, not for heavy users. However, I can imagine many people wearing this watch at gyms, where people tend to look after themselves (healthkit, etc)... but I don't see professionals using it (unless endorsed).
If the battery life doesn't suck (which I fear it does given the total lack of reference to it) and the "iPhone required" isn't as onerous as it seems, I might well wear one just for the easy 24/7* heart rate tracking. Granted the optical ones aren't as good as the HRM straps (as best I remember the dcrainmaker reviews) but they're less onerous for long periods.
I think it's a general limitation of the optical heart rate trackers that they aren't quite "24/7". At least on all the Android Wear devices that feature one, you have to ask it to take a reading, and it's pretty unreliable if you're moving. I didn't see it mentioned in the Apple coverage, but I assume it's probably similarly limited there?
I would have bought one since that's exactly what I want but they're -still- not shipping outside the US. Think they may have missed their chance now...
More significant than "people into serious running" is "people who want to be into serious running". Fitness products live and die based on their ability to hook the consumer with aspirational marketing. A well-placed fitness magazine "review" will impact sales far more than superior tech specs.
(I agree with your point though - I have a 910XT for the battery life - the Apple Watch would be useless as an activity tracker for me since the iPhone 5S doesn't really cope with 5h+ activities.)