Found Ken Wolf, describing a combination of Glass and Conventional Instruments in his Citation 525. After watching this video and after doing a lot of research on the question itself from a non-pilot looking at starting flight training, I was floored at the level of ignorance I had about what's required to achieve true competence as an Instrument Rated Pilot flying a VLJ:
Pretty clear that Ken, really cares about having Conventional back-up. Especially, for his Attitude Indicator - for which he has not 1, not 2 but three (3).
It was humbling watching him go through the systems so fluently. Drives home just how much work is necessary to become truly competent and how important it is to fully understand how the Conventional Instruments are supposed to work.
He admits (by implication) that the G3000 is better integrated than his set-up. But, he's so competent in his own cockpit that whatever his rig lacks, he more than makes up for with skill, knowledge and expertise on his configuration.
One again, this video brings up the same question in my mind relative to this OP: What happens when/if the GDU fails?
Ken's answer seems to be: Rely on your competence of the Conventional Instruments while troubleshooting the problem with your Glass.
It would seem to me that you'd have a hard time doing that without having at least some Conventional Instrumentation back-up. Having said that, Glass seems to be getting more and more prevalent and I'm really starting to see a divide in the Age of the pilot when it comes to this question. Older and more seasoned pilots seem to favor Conventional at the core with some Glass to help with highly task oriented workloads. Younger pilots seem to favor Glass to handle just about everything.
I am still not sure what I am going to do in this regard, but my gut is telling me to spend time getting deep into Conventional first, then make the transition to Glass, while allowing for Conventional back-up in my Time Building Airplane.
But, I'm still on the fence with the OP's question. With more research/study, I'm hoping to have an "Aha!" moment at some point where everything lines up and I know for sure what I should be doing with respect to the OP's question.
Right now, I just don't know enough. I'm too ignorant at this point. That's gonna change.