You can pick up a usb charger in most gas stations and supermarkets. The same is not true if you forget the aa/aaa battery charger. They also tend to be large and heavy and take up excessive space in your pack.
Except that the overwhelming majority of the time, you only need a spare pair of AAs (or two) not the actual charger. I do have a small light AA charger for touring (which I also need for my auxiliary rear lights), I think I've carried it once this year, and didn't actually use it as I had to bail after the third day of the tour.
Anyway, the argument isn't really about the relative convenience of AA or USB power - I agree that USB is a win for day-to-day use (though I feel much more strongly about this with my head torch or phone than my GPS) - it's about the practicalities of waterproofing. If the GPS receiver can run on its own battery, you don't have to think about waterproofing. As soon as you need to recharge it, you either have to make a USB connection to it while mounted on the bike, or remember to plug it in it when you stop riding. That might be fine, depending on what you're doing and what other tech you're using, or it might be an unwanted faff.
And as S2L says, devices like the wahoo/edge units fit to the handlebar mounts very well.
There's nothing wrong with the Dakota-style mount. It holds it securely to the handlebars, and the back of the unit is comfortable in your hand, which matters if you also want to use it on foot. I agree that the smaller Edge units are a more handlebar-friendly size (this is most notable with tiller steering, the chunk of an eTrex is fine on an upright, an Oregon is a bit unwieldy), but that's another tradeoff with screen size and battery capacity.
I upgraded from a etrex10 to a wahoo elemnt bolt, and it has been a revelation. I still don't understand why everyone loves the etrex line so much.
We love the eTrex because the older Edges are barely usable for long distance cycling.
Garmin more generally, simply because they got there first. There are a lot of people with extensive experience of Garmins who can tell you how to make one do what you need. Everything else is a bit of an unknown factor.
I'm going to have to think long and hard about what to replace my eTrex 30 with when it dies. I'd like something that could handle racing better (which might just be a low-end second unit). I need sensible mounting options for recumbents as well as uprights. I despise touchscreens and am completely indifferent about colour. I'd like something that can go for at least 12 hours, preferably more like 18, without having to deal with batteries, but I'm not particularly fussed about what they are. I don't want to have to use websites, smartphone apps or wireless voodoo pixiedust to exchange data, though the options are nice to have, and I'd rather avoid binary file formats. Obviously it needs not to be locked into proprietary mapping, and I'd like to have turn-by-turn navigation and auto-routing available, even if I choose not to use them for audaxy rides; sometimes I just want it to get me to some destination while I worry about the lemming-infested one-way streets, road closures and people driving cars at me.
Certainly keeping an eye on Wahoo, they seem to be gaining ground as Garmin progressively loses the plot.