Having just tried Gaea, I can say it feels super smooth in the 3D view on my GTX 980, and it's only going to get better from what I hear (more Perf Optimizations coming).
I was doing a Terragen rendering on my other workstation that has my better 1080 Ti, so I am going to try that next.
WM Killer? Personally I will keep using WM and GeoGlyph for a long time just because I love collecting, learning, and using creative tools. Heck, I think I still have an old version of Impulse Imagine 3D in my software archive, and it actually still works. hehe So I hope we see WM development keep going on steady now for a long time. But sometimes to effectively compete, small one person software companies need to find developer help. I can't imagine the stress of programming, and supporting an app like Terragen or World Machine all by oneself.
Gaea is certainly the excitement right now though. The energy on QuadSpinner Gaea's Facebook page today was exhilarating (I was up since 3:30am myself).
Derek