I'm fine with the dungeon remembering your kc at all times, but using the door should absolutely consume 40 kc as usual. If it doesn't this change only becomes an absurd buff for last recall...which very obviously doesn't need any buffs. Without consuming kc, players can just kill the main boss, completely ignoring the minions, tele out to heal, tele back, repeat. Especially since reopening the instance causes the boss to respawn instantly.
The kc cost should remain as it would encourage players to stay for 10+ complete kills, which will reward more skilled players for doing successful long trips, OR cause players to actively stock up on kill count so they can rush the boss later, which would be subject to competition for kill count.
Currently they can just teleport out of the non-instanced area and go straight back with Last Recall, avoiding any need to get KC again.
If we were to not include the KC change then people wouldn't be able to use Last Recall in the same way they are now, as they'd have to get KC everytime they want to enter the boss room again unless they used the non-instance version and then we're back at the same problem with overcrowding and people crashing :(