Most likely, expansion features never really stick around in relevance once the expansion passes.
I imagine it will stay around in it’s current form and difficulty however we will progress past it.
You should be able to go back for any achievements and cosmetics you missed though.
That is a question only Blizzard can really answer. I see Torghast as a test of different styles of content, and the question there is does Blizzard think it was a success?
If anything I would imagine in the future Blizzard will use concepts from it that they believe were good, but how they use them is anyone’s guess.
Personally I would prefer to see the cosmetic side embraced with a more true roguelike experience more like halls.
I think there is a potential for it in the game, it just has to be done correctly and probably not as restricted nor as integral to progression.
Release Torghast just burnt a lot of people out I think.