The Fitbit Surge has a GPS built in, gets a week of battery life, is waterproof, and is less expensive. Apple better figure out this who battery ordeal, especially since we are not getting a GPS.
I don't think the trade off solely because GPS is battery inefficient. If anything, looking at how the components are crammed together, I think it's because including the hardware for GPS squeezes out just too much space for the battery, or more than Apple was willing to give up. Not only would the GPS use additional battery power, there would be less in the pool overall, causing performwance problems on other aspects.