Interesting. Other than having a red button I’ve never pressed and some menu items, I can’t immediately think of any negative impacts having video capability brings, especially to performance. Maybe without they could use slightly cheaper processors, but that would be a cost impact.
* different sensor layout, meeting tough demands for video first; stills take second place
* heat dissipation, more heat shielding, bigger body, potential overheat issues
* video processing pipeline/codec - not free of cost, siphons away CPU power from stills and/or AF system performance; more features in firmawre = higher cost than needed for pure stills
* audio: recording - amps + processing power + battery life at the expense of stills; stereo speakers, stereo mics, video-related output jacks [HDMI etc] - higher cost, more difficult to weatherseal; not needed for stills
* UI/Control points/ergonomics: separate "red record video" button (instead of just using regular shutter button) typically not user-assignable, often very stupidly placed/inadvertent activation, wastes precious real estate
* menu cluttering - typically no choice to hide away unneeded video options/submenus; EOS R - no more mode dial because stills modes duplicated for video
and for sure some more. But it is force fed on ALL camera purchasers. I like the concept of Sony with dedicated, video-centric camera model (A7S series), but why is there no "pure stills" version?