Actually, that's not exactly true. Some of the funds collected through the IGNIS ICO will be used to further develop NXT. Sure, ARDOR will be the new main project, but NXT will not be abandond by the developers. Actually they will have 2 different use-cases. But at the end we will see what will happen to NXT. No one knows for sure.
For more infos: https://www.jelurida.com/ico
OK, I stand corrected. And I guess it wouldn't be too difficult to port some improvements from Ardor to NXT or the other way around.