It seems a lot of money for a massive in size AA powered GPS unit... let me see, there are about 3-4 Ah in a pair of Duracell... a charge of a modern GPS Li ion battery is 1 Ah... that means a hell of a lot of AA batteries in one year... could buy rechargeable AA batteries... more money... but then I guess you defy the point of having an Etrex in the first place.
Normal operating procedure with an eTrex is to have a couple of pairs of NiMH AAs, which should be good for a couple of days of continuous operation. If you get caught out, or are on a long tour away from mains power, you can borrow them from your lights or buy batteries from any petrol station or newsagent. I literally can't remember the last time I used disposable batteries in my eTrex.
The main point is that you can carry spare batteries and swap them as needed, rather than arse about trying to charge a USB device on the road, compromising the waterproofing.
Power issues aside, the eTrex is a general purpose handheld navigation device, which means it isn't based on so many assumptions about what a cyclist wants a GPS receiver to do. Whether this is good or bad depends on how you use it, but it seems to me that the eTrex's approach to logging is a better fit for audax than that of the Edge series (you can't forget to 'start' it, for example).
As for massive, handheld GPS receivers used to be the size of a brick phone. Modern ones are *all* tiny.