It's not easy to defend non-user replaceable batteries. Especially knowing that batteries lose their ability to hold a charge over time.
This makes your statement "Devices with sealed batteries last much longer too" difficult to believe.
Of course, no matter what options or solutions you can offer, the fact remains that using a dedicated device instead of my phone means that I'm not using my phones battery for that purpose -- none will always beat out some.
On external battery packs, I've tried tons of the things from the cheap $20 models to the high-end $100 models. I've found them just slightly less than useless. I went through four of the things trying to get my wife's Android phone to last through a work-day before just giving in and buying an extra battery. She would charge one in the evening and swap them out before bed (to have two charged batteries at all times). [In the end, she switched to a Blackberry to avoid the hassle altogether. Though this isn't relevant.]
We're not likely to come to any middle ground on the above, but I would like to revisit a point you made earlier:
Applications on a phone can however reach that status, because software can become excellent in a way a dedicated device fails to reach.
This is where we really differ.
The software is only a part of what makes a device usable. Take my Kindle, for example, eink we already agree is an advantage in that dedicated device, I'll also add that the position and size of the page turn buttons is a big advantage in terms of usability that swiping a screen to turn pages. (Less effort, etc.).
In the case of the mp3 player, it's smaller size, less fragile hardware, and tactile buttons (more than just skip/back/pause/play) mean I can carry it on me in more ways than I can my phone, I'm less worried about damaging it from where it's placed, I can access almost every function without looking at the display. These are all advantages that software simply can't address where the hardware is lacking.
In the case of the GPS, I much prefer a larger screen than the one on my phone. Granted, I like the GPS software I have on my phone -- it's very nice and super easy to use -- but, again, the dedicated device simply suits my needs much better. We can revisit this when we have software-resizable hardware :)
None of these points can be addressed by software. In short, software can never make up for hardware that isn't well suited for the task.
That's my take anyhow.