Really? i thought a 1080ti would be fine for a long time!
Dx12 Ultimate is coming. The features it brings will help improve the graphic quality and the performance in VR games.
Also, with high refresh rate headsets like the Index and high Res headsets like the Reverb G2, you need a more powerful GPU to get the most out of them.
The 1080Ti doesn't support DX12 Ultimate and doesn't have the power for higher end headsets.
EDIT: A 2070 Super will have last longer despite having roughly the same performance as the 1080ti.