Should they stay? Imo yes, because im getting tired of constant character replacing and i actually like Iris and Cilan being interested to have them stick for longer than usual 4 years run.
In reality its not so much about Cilan or Iris having potential and stamina to travel for multiple sagas or not, but more in would fans welcome idea of character sticking longer than usual?
Any character in theory has potential to last for multiple regions whether we are talking about ex or current traveling companions as long writers are willing to handle them and their stories properly knowing in what directions to take them.
Way i see it Cilan has material to work with in case of stay. He is pretty popular ,has several unexplored things about his past and story and if writers carefully expand more on his dream of becoming top connoisseur along with keeping him active through his various hobbies which could evolve in something bigger to compensate for passiveness of his original goal at times over time yes i could see Cilan sticking around little longer.
With Iris from story standpoint she definitely has stamina to last past one region. While some are willing to argue how after Drayden there is nothing to b don with character, but they are largely mistaken because it wasn't clearly established yet how could someone accomplish such thing.
I could see her continuing journey to train harder dragon types and explore more on their weaknesses and how to counter them. I could see her adding new pokemon to her team, her Axew is still in baby stage and with champion position from BW2 games writers have whole new direction in which character could be taken. Such as reflecting such changes on anime in less invasive way, trying to end up as champion apprentice.
However little someone potential and room to grow more matters in this show because faith of characters is largely tailored through games and advertizing. And if past have taught as anything is fact how writers are more in favor of adding new characters from upcoming generations to better reflect promotion of new games, instead of working more with established characters doing build up on their traits and storyline taking them forward.
I guess in theory both Iris and Cilan has around 10% to stay, and that's mostly because writers also showed unusual pattern of behavior during BW series breaking several strongly established patterns from past. Brock departure, removal of contests, main character catching fully evolved pseudo legendary Dragonite, Jessi, James and Meowth turning in competent, serious villains, sidekicks getting actual rivals as well Ash main rival dropping early from league being some of them.
So maybe they decide to break pattern of constant characters replacing too deciding to keep them for one more ride. Unlikely, but not impossible.
Iris will probably leave if they want to go back and having the "generation" main girl as a traveling companion and Dent will probably stay, unless they change it up somehow and make the boy leave this time.
Personally im not quite sure what would give Cilan higher chances to stay over Iris? Brock was in reality exception to rule of characters leaving once generation they originated from is over. So i don't think he is credible indication through which destiny of future male companions can be determined.
Iris is my favourite anime girl but I think the whole Dragon Master thing will get stale if she stays for more the one region just like Kasumi.
Not necessarily ,for how long someone will stay in show doesn't affect quality of character as long writers are handling him properly doing good job out of it.
Even if things start to go on stale note, i believe we can all agree how character can end up enjoyable again if new angle and fresh plots are added to his story.
Now i can understand how some people may get tired of some character quickly. However to be fair many fans continued to enjoy in Ash and other long lived characters who were kept for more than one region being given opportunity to learn more about characters we like, seeing other sides of them being explored more and seeing them going through new adventures and development being given to them. Something which wouldn't be possible to achieve if they were kept for too short.
p.s. Also what do you mean with Misty? Because i didn't found her stale with most of her highlights coming from Orange Islands and Johto. On contrary just when writers finally started to do more with her story exploring on water master dream with Whirl Cup making things become interesting which was fresh breath of air after usual supporting role character was put in, she got replaced.
Most likely is Iris returns to the Gym and decides to train with Drayden. Much like in the games and what happened to Misty, the writers will likely return Iris to where she belongs in the games.
Cilan could go traveling by himself much like May and Dawn do.
Iris game counterpart is champion in BW2 games, so im not sure if gym is anymore place where she would belong?
As for Misty she just like Brock and Cilan had different dreams too, and only reason writers played gym card with her was because they couldn't come up with better excuse to have her leave cast. Hence explaining why she went back to gym out of obligation and necessity. Never mind that her game counterpart kinda contradicts that with statement in GSC of planning to leave position to travel, not planning to stay there.
Either way i doubt writers would be unprepared for Iris leaving like they were with Misty's having departure done in more sensible and planned manner.