Yes. The Buddha Sāsana will end in about 2500 years in the HUMNA WORLD.
However, there will be Ariyas (Noble Persons) in higher realms for much longer.
– In particular, there can be Brahmas in higher realms who are Anagamis for many eons.
– For example, Sahampati Brahma told Buddha Gotama that he attained the Anagami stage many eons ago when he learned Dhamma from a different Buddha. As we remember from “Buddhism and Evolution – Aggañña Sutta (DN 27)” post, higher-lying Brahma realms are not destroyed in a “loka vināsa“, i.e., when the Earth blows up and destroyed.
– Sahampati Brahma says he listened to several Buddhas after attaining the Anagami stage and born in that realm.
So, to Johnny’s question: “can there be any Sakadāgāmi devas leftover from Kassapa Buddha’s dhamma era when Gautama Buddha appeared on earth?”
– Yes. That is possible. But such Devas are not born in the human world and thus cannot bring Buddha Sāsana to the human world. In other words, a Sakadāgāmi deva who attained that during the previous Buddha (Buddha Kassapa) would not be born in the human world AFTER the end of Buddha Kassapa’s Sasana. Only another Buddha (in this case Buddha Gotama) can bring Buddha Dhamma to the humans after that.