I was under the impression that the end of buddha sāsana occurs when noble eight-fold path(and the dhamma practitioners) disappear form all the realms including Śuddhāvāsa where only anāgāmins live.
Imagine a school where only music prodigies or math prodigies attend, one would never have to worry about the decay of the school's reputation. The Pure Abodes are also like that. Since it's a realm where Non-Returners reside, it'll never face the problem of Dhamma decline there. Earthlings on the other hands, are much more inclined towards sensual pleasures and many other unwholesome deeds. It's the Dhamma on earth that's facing a serious danger of going into decline.
In SN 20.7, although the Buddha didn't give an exact number on the life span of the Dhamma, He did give the clear sign and cause to its disappearance:
"...In the same way, in the course of the future there will be monks who won't listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. They won't lend ear, won't set their hearts on knowing them, won't regard these teachings as worth grasping or mastering. But they will listen when discourses that are literary works — the works of poets, elegant in sound, elegant in rhetoric, the work of outsiders, words of disciples — are recited. They will lend ear and set their hearts on knowing them. They will regard these teachings as worth grasping & mastering.
"In this way the disappearance of the discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — will come about.