I don’t believe that to be universally true. There are those who suffer eternally and have no chelek at all in Olam Habah.
The way the Ramchal (ie kabbalistic) explains it, gehenom, while not necessarily limited to 11 months, does not endure forever — at some point, the whole thing is vanished from existence, and the neshamos of people who do not make the “cut” (that’s actually the way he says it) cease to exist together with gehennom. And gehennom is only one of many, many goodies that are waiting up (down?) there for people, but Chazal talk pretty much exclusively about gehennom because it’s the most prominent one (if i’m remembering the reason correctly. I assume there are others in any event anyway).