What sets the RP apart from any other full frame mirrorless camera and most current cameras in general, is that it's meant to be a very basic camera and priced accordingly. It's the 300D all over again, and at a similar price point when inflation is taken into consideration. Compared to the 300D, which was a rather terrible camera, the RP is a do-it-all pro monster.
All other full frame mirrorless cameras, including the original A7, have had at least "advanced amateur" aspirations. The RP cuts through all that crap and presents itself as the basic, no frills alternative. A full frame mirrorless that many can afford and most will understand how to use. Still, it seems to offer decent ergonomics and even a fully articulated LCD. I wouldn't hesitate taking one to use on paid jobs.
Here's a little story that happened two days ago, and that describes how important the RP and its bigger brother may be:
A friend of mine, who owns a Canon 50D with some lenses and a Sony hybrid video/stills E-mount camera with a zoom lens, asked me for advice. He was going to buy a new camera, better than either of the two, and preferably full frame. I actually recommended that he had a close look at the A7 III, since he seemed to be more satisfied with his Sony that the Canon.
Then he went to the shop where I buy my camera gear, also on my recommendation. He quickly rejected the A7, partly because of a user interface that he didn't understand, and partly because he didn't want to use a third party adapter for his Canon lenses on the Sony. Then, the staff showed him the Canon R, and he was immediately sold. User interface resembled what he was used to from the 50D, and his good, old 70-300mm worked even better than on the 50D with the included adapter.
Forget about cropped 4K video, lacking IBIS and limited DR. Most people want something they can understand, and they want to protect their investment in lenses etc. Last time I checked, Canon had a 49% market share in the ILC market (2017). My guess is that that won't change much. They make good products that are easy to use for most people, and their lenses are second to none.