perez is an excellent driver but the time for him to be moving to ferrari is not right atm. the cars are so radically different next year, that ferrari most likely wants a very experienced driver and a very fast one as well as drivers who know how ferrari operates.
ferrari will most likely sign kimi for next year, BUT only for that year. they may even just put perez to be their reserve driver, just so they can hold onto him.
and let's not forget, that even if kimi has still been underperforming this season, alot of it has had to do with the car or bad strategy like in this race (they took him in waaayy too early, he was matchin vettel in pace) and above all, he is still fourth in the championship behind vettel and the merc drivers!
edit. and yes, kimi has been just as much at fault as ferrari and the car have been:
australia: engine failure
bahrein: second, clean drive.
china: fifth, was crashed onto by vettel at the start
russia: third, clean drive. started fouth.
spain: second, clean drive. started fifth
monaco: rubbish in every way, took the blame.
canada: sixth, started sixth. bad conditions for ferrari. bad strategy by ferrari. spun because of the same --- fault as in previous two races at canada, at the same hairpin.
baku: fourth, started fourth. could have gotten third even with 5sec penalty, if he hadn't been taken in too soon.
those are all pretty solid final results, even if there was potential for more. it's only the last 3 races that have gotten away from kimi. otherwise he's been decent and in my books worthy of his seat next season.
kimi is kimi. he's the unluckiest race driver i know, and kinda prone to faults. but he is still very fast.
but even if force india sign perez for multiple seasons, ferrari has enough money to buy him if they want.