Its not a forfeit, its one of the chief perks of Kamui, something Obito takes advantage of all the time. Who makes up these rules? Who decided if Kakashi goes to Kamui dimension he automatically loses? It seems like an excuse when one has no more legs to stand on. Unless Kamui is restricted in some way in the OP (which it is not in this case) then it still stands and Kakashi can use all his Kamui abilities.
BFR
OBD Concepts
Short for Battle Field Removal or By ****ing Ringout
BFR is the tactic of defeating an opponent by expelling them from the battle area, so that they cannot return under their own power within a reasonable amount of time. Like Speedblitzing, BFR is often not used by characters who are capable of doing so in actual storylines, often due to PIS. However, in the OBD it is considered a legitimate tactic.
Examples of BFR:
- Trapping someone in another dimension
- Burying someone underground (if they can't break out)
- Sending someone to a different time
- Throwing someone into space (if they are stronger and the enemy have no way to return by themselves)
- Throwing/hitting someone so far away that it would take an unrealistic amount of time for them to return
So no, Kaguya sending Sasuke to the Ice dimension is not a forfeit, its a legitimate win. Kakashi going to the Kamui dimension is not a forfeit either, he has the means to bring himself back anytime he wishes and like I said, its not like hes going to sit there, he goes for just a sec, comes back to real world in Sasukes blind spot and snipes him. Game over.