I've been using a borrowed A7R3 for the last few days, and I get why it's so attractive. It's a camera that promises to take beautiful pictures, faithfully recording what you see in the viewfinder and not requiring you to do much else. In other words... a perfect tourist camera.
The sensor is fantastic, but it actually isn't much different from a 5D4 in practical terms -- or even, gasp, a 6D2. Don't let that 43 megapixels fool you -- there are precisely zero pictures that were incredible, that wouldn't also have been incredible with 30, or even 26 megapixels. You can't really deeply crop amazing pictures out of that 43 megapixels any more than you can out of 30 -- a small object in the distance might get a few more pixels, but it's still too small to make a good photo. People crow about the dynamic range; in fact, you can't really correct overexposures by very much (same as 5D4), and lifted shadows do not look anywhere near as good as correctly exposed photos to start with.
On the other hand, the Sony has a horrible flash system that is infinitely inferior to even Yongnuo's knockoff RTs, is incompatible with pattern-style AF illuminators, and has only focus-by-wire lenses, and is painful to hold for more than a couple of hours without a egregiously overpriced grip. Battery life is "fine", except it's not, because you'll easily go through two or three batteries if you need a full day of shooting. The finger dials feel cheap, and the shutter button is set so high up with a ridge around it that it's impossible to gently depress (so as to not add motion to a large lens). The weather sealing is a joke.
It's a camera that's first class in many respects, but falls really flat on some key things that I think that a lot of photographers would value.
So why is it so popular?
In large part, I think, because Sony caters to a segment of photography enthusiasts that don't care about the things that it's not good at so much. I mean, I have met many people with $10,000 in their camera bag, who don't own and never want to use a flash. There are people who think that telephoto ends with 400 + 1.4x. Or who would never consider using a tripod and a long exposure. Or use an ND or polarizing filter.
They are dismissive of the reasons for the size and bulk of a DSLR, and really just want something smaller that is easier to use and that is technically capable of producing professional-quality output.