I agree with you the space elevator wouldn't be workable with steel. But I don't agree that carbon nanotube material is in the "unobtainium" category. The key characteristic of the space elevator material will be its strength to weight ratio: only giving a materials Gpa is only giving part of the story. Calculate the strength to density ratio for steel compared to the Zylon material you mentioned, then do your calculations for Zylon. Carbon nanotube material would give even better numbers.
Carbon nanotube research is proceeding apace. I estimate we will have commercially available nanotubes within a decade. Then give another 10 to 20 years for engineering and technical expertise on its use. So space elevator possible within 30 years.
That's a different question though from whether it would be economically feasible or useful at that time.
I'm guessing there will also be other easy space access methods available then.

Bob Clark