... But would you go as far as honestly advising newcomers to buy a 5Ds? I’d be really interested in the rationale...
If someone just crawled out from under a rock (that is, completely new to photography, say coming from a smartphone camera), I would not suggest 5Ds. Having said that, I wouldn't suggest D850 or Sony 7 or 9 series either. Too sophisticated for a newbie. But if you (or they) insist on immediately jumping to the best, I would suggest to handle all three cameras, maybe try it for a few days, to see how it feels.
I once wanted to buy a Leica R (reflex), tried it in a store, and immediately disliked it to the point of abandoning the purchase idea. Then a few years later I tried Leica S and immediately fell in love with it (in contrast to the R model, I couldn't afford the S).
Furthermore, the newcomer you mention would need to tell me what are his intentions with the camera. What type of photography? How big the prints? How many lenses? Which type? I bought a 50 Mpx camera, for instance, because at the time I was selling really big pictures in art fairs, and I already own plenty of Canon stuff.
If the newcomer is not really a total newcomer, but is already invested in a certain ecosystem, would need to know how expensive and time consuming it would be to jump the ship.
But let's say I lose all my equipment in a fire or is stolen and I get insurance money fully. I would first reevaluate my current needs. Do I still need big prints? If no, I might downgrade to m4/3 for portability. If my needs remained the same, I would give a chance to Nikon D850 (I still prefer the optical viewfinder), but not until I try it for handling. I would also try Sony for the same. Depending on my mood (long-term one) I might be too lazy to learn a new system and would stick to Canon.
5Ds today is rather old, so I wouldn't buy another one today. If I would stick to Canon, I would probably go the mirrorless route, perhaps waiting for the pro body. The attraction? The new 70-200/2.8 which is significantly more compact than the old version, as well as the new crop of lenses.
Does that answer your question?