With the weight you're right, it's absurd - but most of the Art lenses are hefty.
Back to the 50 Art. I will not exchange it for the much lighter 50/1.4G, in fact I got rid of that when the Sigma came out. Sharpness wide open still is incredible, so for me, if it has to be a 50 mm, i don't see a point in constantly thinking "neat, but with the Sigma it would have been really good" I can't speak for any other 50 mm Art, but mine is close to perfect with center AF on PDAF.
Against the unreliable focus: I'm afraid the medicine's comes in a package with a big SONY logo and the number A7RIII on it.
Okay, neither the Sigma nor the Sony are something for toni's definition of "winning team", but others might find it useful to know.
With the lenses such as 135/1.8, 20/1.4 etc., I can tolerate that weight. It's a massive thing but it's supposed to be that way. And it lets me do things other lenses won't. The closest thing is Canon's still magnificent 135/2L and I'd seriously be thorn between that and Sigma. Actually, I'm still keeping an eye on Batis 135/2.8 too. I know it's hilariously expensive but the quality is there, the lens is compact and I'd rather have a native lens on my A7.
As demonstrated in my last sentence, lenses don't exist in vacuum, nor do the cameras. Since I'm already shooting Sony, I got the 55/1.8. That thing is a little gem and for all my intents and purposes, performs as good as Sigma. At about a quarter of the weight and size. And at the same price (I actually got it for quite a bit cheaper than a used Sigma).
Would I also get the Sigma if I were shooting Nikon? Not so sure. Maybe I'd go for the 50/1.8G. It's good enough for me as I'd probably have a 24MP body anyway. The intended budget and quality expectation for the Sigma would be focused somewhere else, maybe by getting the 105/1.4. To be honest, after a long time of shooting film, I'm not that much of a pixel peeper. The only time I need the absolute quality is when I'm doing the professional side of my work (architecture and interiors photography) and I don't really need a 50mm there.
If I were shooting Canon... I'd switch. Honestly, there wouldn't be any reason for me to stay in Canon. Except maybe if I were shooting birds.