Whilst I'd love to be wrong. I can't see Apple struggling to make the watch last for days and days on a charge, they would rather make it thinner, faster, add more things to it, than add days of battery life.
For me, since the watch is something to be worn all day long, I'd rather have a thinner and lighter one that has to be charged every day, than a larger and heavier one that will have days-long battery life.
And I know you'd say Apple should then make both, so people can choose. But if you value choice, you are with the wrong company. Ever since Steve Jobs returned to Apple and pruned down Apple's product line to four basic computer models, Apple has been about simplified product lines. They don't make devices in one-inch size increments, and they aren't going to make watches in one-gram weight differences.
I respect that people don't agree with Apple's philosophy, that some people think the right thing to do is make phones in one-inch size increments so people can choose. But Apple thinks that it is their job to make that decision, so people don't have to choose. Just look at all the people trying and returning their iPhones 6 and 6+ not being able to decide which phone is the right one for them. Now imagine the amount of confusion if there were, say, five sizes to choose from!
Yes, in a way, it is arrogant of Apple to make the choice for other people. But on the other hand, they are the engineering and usability experts, they should know better than the average person what works best. So if you trust that experts usually know better, buy from Apple. If you think you can decide best what works for you -- well, Apple may not be the right company to buy from.