I'd like to suggest to make mission NPCs stop or at least appear less frequently in the battle tab when there is enough mission artifacts accumulated. Or maybe give them a time limit to disappear would also help.

The reason I'm suggesting this is that there are missions that requires artifacts to finish and can take a very long time to finish, at the same time those NPCs will keep on appearing and forces players to disable them in order to have a different NPC take their place.

finish the mission and they go away.

Yes, I think for now, I'll just do missions instead of battling NPCs.

As mentioned in another forum, some missions with NPC's are not economicly viable. You could have full mission NPCs in your battle tab when the mission itself will take weeks to complete. I suggest that mission NPC's are given the same timers as all other NPCs of the same type, with the exception of "boss" mission NPCs. (such as the Stryll Deep Tracker or the Zolazin Shroud Base)