1. People like business travelers or those with even minimal levels of status/benefits (who don't pay for checked luggage) don't usually preferentially check bags because luggage gets delayed, it's harder to switch flights when there's a weather etc. problem, and they have to wait at the luggage carousel.
2. Hard and hard-ish roll-aboards are a menace. Especially in a world of generally more casual dress, soft-side luggage would make overheads a lot more manageable--understanding that some people really can't use shoulder bags or backpacks.
On vacation I don't have my work laptop, so it's easier to toss toiletries and an emergency change of clothes in a small under-seat carry-on bag. Besides, tourists aren't expected to smell nice and look put-together, and are more likely to have a flexible schedule that would let them go shopping if the bag doesn't turn up.
Only once has the airline lost my bag while on vacation. It was only slightly annoying and they found the bag and got it to me eventually. I've seen a coworker whose bag was lost on a business trip to India. He was stuck wearing the same clothes - a tshirt and jeans - for multiple days. This included time in the office (which had a dress code) and at least one business dinner.
Not that taking everything carryon was really an option in this case, but I had a bag misplaced after a connecting flight was canceled. This was a group hiking trip but I had at least an extra day scheduled. Still spent about $500 to minimally restock although my bag arrived at literally the last minute before one of the guides left the hotel for our one-way walk.