Having been on some flights of other international airlines that didn’t offer pre-boarding.... it was actually much worse if you can believe it.

They boarded their elites and priority boards first, and then some of the pax with needs (who would qualify for preboarding in DL) started to board. You had elderly people in wheel chairs, parents with car seats for infants/toddlers, etc. all trying to get in with the rest of the other pax. It was a mess.

Watching the elderly with mobility issues trying to get around other pax who were already seated, it just made everything grind to a halt. Then you had the parent trying to figure out how to assemble their car seat in a row with another passenger in the aisle seat, who had to get up from their aisle seat and stand in (and block) the aisle while the parent took a few minutes putting it together.

Long story short, it was bad all around. And it gave me a new appreciation for preboarding. Even if I am flying F, I want the wheelchair people and infants to get on first while the plane is empty. After observing that, it was obvious that preboarding is better for everyone.