Great discussion. I agree that one cannot really approach this without thinking holistically... the entire analog playback chain should be considered a single, musical biosystem.
That said, I do believe that one should start from the foundation of the analog chain, which is the turntable. Cartridges and tonearms should really be considered inseparable, but they are variables more easily accommodated and changed physically. The turntable, less so. Once you have a solid foundation (a TT that you are satisfied with), it becomes a bit easier to discern among many tonearms and cartridges to match with the TT via a natural process of elimination.
Yes, a great phono stage is crucial, but again, selecting the right phono stage to the turntable/tonearm/cart seems a more natural process to me than the other way around. More often than not, people would look for a great phono stage which would bring their TT to greater heights... I've never personally met someone who told me, "I have a killer phono stage and now I must find a suitable turntable to match it"; it's often the other way around.
So for me, this isn't a chain of importance, but of PRACTICAL consideration, which was my personal course:
1. Turntable
2. Tonearm/Cartridge
3. Phono stage/SUT