I don't see the advantage, why would I pay the size / weight penalty? I don't use my machine long enough on battery in a single sitting to gain any benefit from extra batteries.

> universally better keyboards (at the very least, more robust!),

That is not an objective statement, its purely subjective.

> usually far cheaper for far better internals,

Not really, if you actually build a comparable machine the 'apple tax' doesn't even exist for many of their product, and is a couple hundred bucks for the rest. I spend a significant portion of my life on a computer, a couple hundred bucks means nothing. What you can do, and what usually happens when people do cost comparisons, is cheap out on some parts and come up with something considerably cheaper. That's fine, but it's no longer an apples to apples comparison and not a matter of Apple charging too much.

> ability to swap hardware out, before-market and after-market customization options...

Hardly anyone does this, even for desktops. I've been building my own PCs for ~20 years now. I remember the days of the 440BX chipset, when it came out you may have had a ~250Mhz cpu and 128MB of RAM, but just a couple years later you could toss a ~1Ghz PIII and multiple GB of ram it in cheaply. Thing is, that doesn't happen anymore. CPUs aren't advancing that fast and by the time you would want to upgrade the CPU there is a pretty good chance the new CPU requires a new chipset. RAM is also stagnant, we aren't seeing higher density or cheap prices (quite the opposite on prices actually, they have sky-rocketed). Point is, the last 3 PCs builds I've had are fully expandable and everything is swappable, yet the only component that has changed in any of them is the GPU. Expandability just isn't terribly important anymore.