I have a "reverse" thought on this topic...

I think the convenience of a single bag that carries everything — the laptop and all the rest is key for convenient travel. It is much easier to navigate the world with just one bag. 1-2 day trips shoulder bag, longer trips rollaboard for me.

So far, at the destination I have used a thin strong cloth light shoulder bag (Sea to Summit Travel sling bag) to carry my laptop (in sleeve), power supply, demo and presentation stuff to meetings etc. But this is not substantial enough to be used on plane alone e.g. under the seat.

Recently I have been thinking of getting a laptop-bag-looking-bag just to take advantage of the extra baggage allowance of some airlines, i.e. a "personal item/laptop bag" + normal carryon bag. I would not use it for my laptop but other stuff ;-)

Thus I have been looking for some bag that looks suitable for a personal item and has a some structure or protection but folds into very small/thin space — could be used as extra carryon volume when needed but pack into very little space in the normal bag otherwise. Haven't found a good solution yet, most bags are way to large/heavy/thick.