Yeah, the other challenge with GNUstep’s adoption is building a desktop and an app ecosystem for it. There are some GNUstep apps, but the ecosystem is not at the level of KDE/Qt and GNOME/GTK. In addition, even if GNUstep becomes fully compatible with the latest version of Cocoa, there are many macOS APIs that are not part of Cocoa. Another complication is the Mac ecosystem’s gradual move toward Swift.
Haiku may not have a dynamic API like Cocoa/GNUstep, but it already has a well-designed desktop and it is capable of running BeOS binaries on x86 (but not x86-64), IIRC. From a desktop perspective, Haiku looks very promising; it’s finally almost ready for prime time.