The range and quality of optics have been the biggest factors for a committed photographer (buying more besides a basic ILC+kit lens). Options in a lens system has always been the No 1 Decider for me. This includes options in Used lenses (i suspect for many of us): as even a "few" lenses are a major investment. This the leveller/reality check (not least with a spouse!!).
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This thread gathers a most interesting range of responses/justifications. One trend is the attractions of key camera vs key lenses; some of us are attracted to the 'best' current camera and/or lens(es).
We can expect shifts in products emphasis - and R&D - as ILC production shrinks, and the squeezing of revenue ramps up rejuggling of company strategy to shift focus to prosumer markets. At the same time, the image quality out of these modern cameras has also levelled off. Their IQ is universally excellent. There's not much left to improve. Video functionality will be refined sooner than later, and so will seamless WWW connectivity (share image as simply as a smartphone).
So what major factor (if any?) in a new camera will enticing upgraders? Either even lighter models with advanced battery power, or more releases upgrading in the "Do it all D850 style" genre? Perhaps stacked sensors will be the No 1 leveller; if so their high production costs will maintain the uniqueness of this niche product, and reposition the companies innovating these more expensive cameras? The market for stacked-sensor ILCs seems limited ie action genres only, because 'standard' sensors work already very very well for the vast majority of photographers (still using an ILC and not a 'simple' phone).
Nevertheless, I for one refuse to buy another camera that fails to include a minimum set of controls (Custom especially). This will have to improve on the minimum "haptics standards" of the Triumvirate (ie D5/D6, D500, D850).
It is obvious lenses is where we can expect more growth in innovations. So sooner than later, yet again, it will be the lenses determining the system in which the returning client invests. So it will be interesting to consider the line up of these R, L, E, and Z mount systems.