Some random thoughts,
I remember seeing shamisen and koto players introduced as holding such and such dan grades in their respective arts - which was eye opening at the time. It seems to be an accepted nomenclature for the arts in Japan...think its the same for tea ceremony, brush arts and even mochi making too...why not aikido.
In the quest for codification and structure, and where endeavours involve so many people that implicit understanding of who is sempai and who is kohai doesn't exist. And the inevitable conversation about when were you born when did you start to practice aikido (or anything else) and who is your teacher in order to discover who is the sempai and kohai is time prohibitive. Imagine having this conversation everytime you paired up/ lined up/stood in the food queue with someone new at a seminar in order to discover who gets to attack first, then having a kyu/dan system makes perfect sense.
A few years back I visited Tsukuba University, it is famous for its sports and sports research. Who's who in Japan and sports research seems to be an old boy from there. Wasn't all that surprised to discover that this was the eventual derivative/successor/ amalgamation of Kano's educational institution
The Kyu-Dan system has spread into all kinds of thing in Japan - I don't see the argument "everybody is doing it" as very compelling, but that's me...
I very rarely have any of those conversations about who is sempai and who is kohai except, perhaps, as part of the general conversation. It very rarely gets you any useful information, and there's so little consistency with ranks that hearing about somebody's rank is virtually meaningless these days.
Much better just to touch hands and see what happens.