I think digital photo tech is approaching maturity.
Good-enough-for-most-purposes images can be captured by everything from iPhones to Hasselblads and Phase MF.
So increasingly your choice of camera systems is going to be driven by your own artistic vision, and whatever requirements your clients, customers or final purposes for the images impose on you.
You can probably shoot billboard-sized images with an iPhone - someone has probably done so. (But I bet they had studio, lighting, art directors and so forth backing them up, like the film Tangerine, shot on iPhones which recently triumphed at Sundance. They put all their money into other areas of the production).
You can shoot website photos with a Hasselblad. I know, because that's my day job. Could I get away with shooting everything on my iPhone instead? Yes, but the results wouldn't be pleasing to me, and not to my more discerning customers either.
If you shoot billboards REGULARLY, you'll doubtless find it a lot easier with a 5DIII, Phase or Hasseblad than with an iPhone or bottom end m43 compact.
If you shoot sports regularly, you'll find top end dSLR's from CaNikon offer performance above and beyond the capabilities of other systems, for the moment.
So it is all about how easy it is to get the results YOU want in YOUR workflow with each size or style of camera.
I have a Panasonic GH4 system, and while I love its flexibility, light weight, ergonomics and battery life, it cannot be denied that the detail and tonal richness present in the shots
I take with it in the mountains is sub-par compared with my Sony A7Rii. So while I have taken lots of pics I love with the Panasonic, I no longer take it into the mountains. I just like the Sony's renditions better. Another photographer might find the absolute opposite, or find the extra weight of the Sony system or its crappy ergonomics or poor battery life a compromise to far.
For me the compromise too far in the mountains is the Hasselblad- too heavy, too fragile-feeling. Of course, there are many excellent photographers who use their Hasselblads for landscape and mountain work- and many who use m43 or APS-C systems, too. It's personal choice to improve the number of "keepers" you get each trip.
In the studio with flash lighting the tonal richness and detail of the A7Rii is a bit behind the Hasselblad to my eyes. The detail gets close but loses out in terms of artistic preference, and the colours are definitely easier to get the way I want them on the Hasselblad. Conversely, the Sony in available light with 85 mm f/1.4 GM lens absolutely kicks the Hasselblad's ass in terms of sharpness, camera shake reduction, and hence detail. And the bokeh is luminous.
For another photographer with different subjects and different workflow and different artistic preferences, they might prefer the A7Rii in the studio. Some will prefer Fuji's rendition, some Panasonic's. Some are so attached to Canon skin-tone rendition that they'll absolutely accept the marginally smaller dynamic range and plump for the 5Ds or 5DIII. Or they might have a need for lenses which only the big boys have in their ranges.
Some people may even like Nikon's skin tones
(I'm sure they're great in skilled hands, but I dread people turning up with Nikons for people photography tutorials- to me they seem to render everything glowing plastic orange and purple.)
So it is all about which compromises matter to you, and finding the system or systems which best suit your tastes, your personal vision and your workflow. It is possible to take extremely good photos on pretty much every modern imaging system, so pick the one which works best for you.
These days there will be factors which will matter MUCH more to you and your workflow than just the size of the sensor. Hire kit to figure out what they are, and buy accordingly.
Cheers, Hywel