There is a market (not sure how big, but there is one) for modern-day luggables. They're more often called "desktop replacements", and "working without a power source" is really far down the list of requirements for them.

The common use case for these machines: a developer who needs a fast CPU and lots of RAM (and often a dedicated video card) coupled to a 15" or better yet a 17" screen and a ton of hard drive space. You may unplug to go to a meeting for an hour or two, but you'll be back to your desk (and docking station) pretty quickly. You're not working from a coffee shop or a couch because you need a real mouse and a real desk. You occasionally take your work computer home when you're the on-call resource that week.

In that case you don't need a big battery (or rather you need a big battery but not a lot of battery life). Five hours is more than good enough, but you can't trade that performance for anything you don't need.

I know because I have one sitting unused beside me (Thinkpad W530) from back in the days before I started traveling for work.