You've always been able to gain additional battery life by adding weight and cost. The remarkable thing here is that it has been done by adding neither. For the overwhelming majority of users, there is no downside to this. For the rest, it is an inconvenience at most.

Well, the battery isn't replaceable, so there is a downside. You can not get more runtime by carting around batteries (unless you jerry rig a 15V battery and the airline adapter...)

The overwhelming majority of users never replace the battery on any system anyway, hence as I said it is not really a downside to them. The minority that do is perpetually shrinking as individual battery life increases.

You can not get more runtime by carting around batteries

If you insist on the batteries being internal, then yes, but if like the parent post you are already willing to spend the money and cart around the weight, you can just buy one (or more) of the many external battery packs on the market. The portion of users for which even this is unacceptable compared to carting around 2-3 spare internal batteries must be staggeringly small.

3 batteries? you're crazy. http://www.seatguru.com/ + airline power adapter = win.

What airline are you flying LA-Paris that doesn't have outlets, even in economy?

United has no outlets in economy at all, afaik.

Air France? There were no outlets in economy on CDG-SFO flights (last checked last year).