That is sad news. And lame. Well, if Apple blows it on the next macPro, then I may end up switching to Windows, and then I'd probably use Cubase, but that is probably years away, hopefully never because I don't want to leave OSX. I will go back to my regularly schedule LogicPro and DP challenges. I guess Cubase might work for some stuff so I'll mess with it now and then, but man...it is literally double the CPU use of Logic playing back 100 tracks through VEP. Where Logic is humming along at 20-50% to play it, Cubase is peaking the 90's. Too close for comfort. I haven't even tried any synth plugins in it yet.
I too was on that same boat. Had a spec'd out Mac Pro and Cubase felt poorly optimized (I even contemplated switching to LPX) Cubase worked okay on macOS but thankfully I also have a fully built PC as one of my workstations. Once I transferred everything over, it was night and day. As well as having Thunderbolt 3 on my PC, it all made sense. All of my servers are running Windows as well as my main host. Absolutely no issues. Overclocked my host a bit too
I use my Mac Pro as a Pro Tools only machine now. As Pro Tools just runs better on macOS I feel. Same kind of story as with Cubase on Windows.
I'd suggest switching to a PC if you enjoy Cubase/Nuendo. Otherwise go with Logic Pro X. The optimization on macOS is what makes Logic so great.