I would settle for 3 days
I think this is just the 80/20 split. Almost everybody goes home and sleeps almost every night. For the vast majority of cases, taking your watch off to charge it once every three days is no better than taking your watch off to charge it every night. And the tradeoff to get to three days is either a) a battery three times larger, b) a watch that is three times more power efficient, or c) lesser capability. A three day battery life isn't worth the sacrifices you'd have to make to get it.
I don't want to charge a watch every night!!
Why? What's so much better about taking your watch off every three nights instead of every night?
Actually, something that you don't do every day (at the same time) you won't build a habit around. I kept forgetting to charge my Pebble and so didn't use it much for the first few months (well, to be honest, not really until iOS7 and better notification support) - one of the big reasons is I didn't have the habit of charging the device every night. So I'd forget on day 4, then day 6 it would run out of battery and stay unused until the weekend when I'd be like - why TF did i buy this thing?
Now I charge every night. And I use it every day - it helps me not miss calls (phone is always muted b/c meetings) or important texts (did you know that some daycare centers charge like $1/min for being late? If wife can't pick up the kids on time, I better not miss the call/text/voicemail).