Sure, people often travel without checked luggage. But the "basic economy" tier typically also forbids you from bringing a carry-on bag (just one personal item)

Sure, people often travel without checked luggage. But the "basic economy" tier typically also forbids you from bringing a carry-on bag (just one personal item), and you also don't have a reserved seat. How often do people fly with just one personal item? And if you don't have a reserved seat, then you're the first person to be bumped if the flight is overbooked and everyone actually shows up. So while some perks are often optional, most of the unbundling affects core features. And presumably the reason for this is so they can show up as the lowest price, get you halfway through the purchase process, and convince you to upgrade.