This is purely a marketing perception. My 5Ds has taken over everything...including sports, wildlife, and airshows...and the only thing I find wanting is the RAW buffer depth. It's even handling astro very well. I'm not sure why Canon pitched it as a 'narrow use' camera other than to preserve the higher USA price, or why so many reviewers presented it that way. There weren't any real differences (for stills) between it and the 5D3 except of course for that gorgeous sensor, and it handles every photographic situation well. Someone might point out file size or processing times, but you can always shoot in 28mp mraw.
Those looking for a true 5D4 equivalent mirrorless for stills will likely be satisfied by the high resolution R if they just use mraw when they don't want the full 83mp. The drawback will be the price, not a use or situation limiting spec.
Now for video I don't expect the high resolution R to be top of the line. It would be nice to be wrong here, but even the Sony A7r's have lower quality video than their 24mp brothers. It's just too much data to scale down at high quality. But for stills the 5Ds/sR are not actually narrow use cameras, and I don't expect the high resolution R to be either.