So how far and for how long do you think someone can fly with a luggage that fits under their seat?
You can't honestly say you're selling a transportation service and let people take almost no luggage.
If a fee is unavoidable, then it's not a fee, it's part of the price.
I can't speak for everyone (and are probably an exception, since even for a 3 week vacation my luggage never exceeds 15kg). But I can easily go for a very long weekend (up to a week) with the contents of a small duffle bag that easily fits under the seat.
Which means a fresh t-shirt, underwear and socks for the entire time. It includes toileteries, something to read and a medium sized camera.
In my experience people usually pack far too much.
Seems more of an edge case than the standard. Especially with limitation of liquids in handbags.
A toothpaste tube, deodourant and a mini shaving foam fit easily in the 1L limit.
Most business trips where you're away for at most one night. A long weekend city vacation is also easily doable with only that much luggage.
In any 1 week vacation to any destination around the world? People are diverse and you shouldn't presume everything about yourself applies to everybody else. I've been on dozens such vacations.