18650 are different in voltage that your classical AA batteries, plus packing more energy into the battery while increasing the cost makes little sense when you can simply buy rechargeable ones and use 4 or 6 of them to have one set always ready. Maybe it's just me, but talking about the extra cost of a few batteries and a cheap cable probably every single one of us is having laying around while discussing a device in which you plan to store thousands of $ equivalent seems a bit of a push.
While Lithium batteries tend to hold good charge unless they're exposed to cold environments or obviously used they hold their charge decently well. 18650s aren't all that expensive these days. When you compare them to double AA's sure, they're comparatively cheaper, but if you're using your hardware wallet semi frequently, running into charge issues would be annoying. For a little extra cost, you could have two 18650's instead of three normal double A's, and you'll have a much better charge.
While 18650s are usually associated with flashlights, and battery packs I don't think they should be limited to that. The cost of them is going down, as we are developing stronger equipment that requires more mah. Therefore, double A's I can see becoming a thing of the past, and while they're brilliant for remote controls, and the like. 18650s future proof the design, and when you're storing Bitcoin for a rather long period of time having that future proof insurance is nicer to have.
Personally, I think hardware wallets should accept several different batteries. I have a DIY battery pack, that I take traveling which accepts double A's, 18650s, and 21's. If they did that, it effectively future proofs it, and allows quick access to whatever the user can get their hands on, if they require to do that in a short period of time.