Lets face it only reason why people give Cilan higher chance of staying than Iris has is because writers already did exception once with male sidekicks when they kept Brock. But if we are going to talk about making exception to rule, than someone could argue how Iris might be "exception to rule" as well.
Way i see it Cilan chances to stay aren't really promising at all. Of course in theory he might stay but past events doesn't support this theory much at all.
Lack of development and room for further growth? Irrelevant because majority characters which left cast in past had potential to be done more with no one accomplishing his dreams except Tracey. Writing staff pretty much made it painfully clear how character development and doing more with some character creating continuation isn't on list of their priorities. Horrible way of thinking but cruel reality is how advertizing was always infinitely more important than development in pokemon.
To add stability to main cast treating pokemon series story as more cohesive entity? Writers flushed down the toilet this idea when they replaced already established male sidekick who was kept for years for apparent continuity purposes(Brock).
Logically it doesn't make much sense to replace long lived established companion like Brock after more than decade to bring in another guy who will be kept for years. Fact that they did indicates to me how they will try to apply same approach when it comes to male characters like they are dong with girls starting to constantly replace them joining rest of crew of long forgotten and abandoned companions fading into oblivion.
Popularity doesn't mean much either because ton of highly popular characters got replaced already.
In nutshell theoretically Cilan could stay to balance constant cast cycling out and provide additional familiar face to who people could relate associating series with, but right now all factors ad patterns from past especially after Brock being kicked out despite long stay) are pointing in opposite direction.
So while i wouldn't mind Cilan staying if anything because i would find dreadful to have male companions being replaced so often like females are, in reality his chances aren't very good at all.
For stability and continuity purposes, it's better to keep cilan than bringing in two more new characters. Cilan has been here for almost 3 years and he hasn't done much other than cook and bring knowledge. There's no reason why he can't stay.
Ironic thing is that same argument can be used for girl characters stay.
We have so many great companions which have potential to offer more. There are so many new ideas and unexplored directions which could bring new life to Misty, May, soon enough Cilan, Iris etc reinventing and adding a whole new interesting note to them.
And its sad that because of writers laziness we don't see them investing more in someone who proven to be popular coming up with innovative ideas to keep characters interesting and fresh, making us learn new things .Instead of focusing more on one character building up on its story, they are rather introducing constantly new companions giving us fragment of half developed characters which leave unfinished despite having potential to offer more turning anime in mess which lacks cohesive story, have very little continuity and highly unbalanced cast having no brand to which people could associate series with.
Iris has a higher chance of staying in the main cast due to her goal being far off.
From story standpoint your right, with Iris dreams having technically enough material to last for extended period growing on bigger things. Unfortunately story and development doesn't influence character stay at all. Misty had goal of becoming water master having room to be done a lot in that unexplored area, but writers still replaced her leaving unfinished job. Same applies to extent to May which had more to learn about becoming top coordinator etc.
Misty stayed for two generations, but she didn't go beyond the original series, which encompassed both. She left when it ended. That's the point: each girls lasts only one series, and I highly doubt Iris will stay unless the next generation/region is still the Best Wishes series.
Although originally writers intended to keep Misty as permanent female protagonist, which explains why she lasted a bit longer than other girls did.
Idea of constant character replacing wasn't born until middle of Johto.
Iris story arc is pretty much done though dispite the fact that shes not a gym leader yet, Dento like I said isn't even close to achiving his goal and hasn't gotten much attention as of late,
I doubt this would stop writers in keeping or replacing characters. Since they can always if want do sequel to Iris story or just leave things unfinished about Cilan to make room for new companion.
It wouldn't be first time they did such thing.
I remember when people thought dawn and may were going to stay, look how that turned out?
Don't forget Misty, most didn't expected she will leave at all back than.
