That comparison you posted back then is quite impressive.
To be fair, that is a 1993 film era lens (and not a particularly good one), already struggling on a 12mpx 5D classic, ported on an extremely recent 30mpx sensor against a 30yo newer lens, the difference was to be expected in relative terms; then, in absolute terms, I agree that seeing that much of a spherical aberration wide open is simply astonishing (exacerbated by the comparison with the 40 Art which is a razor).
But even at f1.8 against the 50 STM the spherical aberration is still unbearable, while the 50 STM is already very sharp at its widest aperture; a last generation high res demanding sensor like the one in the R5 would have been even worse.
I have tried the "small" STM triad (35, 50 and 85) on my R6, so relatively low pixel count, and that's what I got:
35 STM for me is the "one lens to have" in the system if I had to choose only one prime lens to purchase: it's light, bright, sharp even wide open, IS works wonders, AF is fast enough and precise, half macro is a very welcome bonus. But I was rarely using it, the 50 Art was always in my bag. I ended up selling both 35 STM and 50 Art to get the 40 Art which is the perfect middle ground between the two.
50 STM does its work, period: for the 130€ used I paid is plenty bright, plenty sharp, plenty fast AF, plenty pocketable and always in my bag at every wedding, but it's there as a backup for the 40 Art. It's not a dream lens by any means, but it's cheap enough to sit unused in my bag, and good enough to do the job in an emergency. That's all I can ask it, and it's the only one I still possess today.
85 STM definitely the sharpest, I really really REALLY liked the images that were coming out of it, a razor wide open with a very nice background blur. And the half macro is even more manageable then the 35 STM. So why I sold it after just six months? Well, the AF. The AF is absolutely DREADFUL, it's slow as f*ck, and it's hunting as f*ck in almost any backlight situation, which unfortunately is my preferred way of using a portrait lens in location.
Also, I was coming from the Sigma 135 1.8 Art (a beauty!) and wanted to buy a more manageable lens with indoor distances in smaller venues, but I couldn't "connect" well with the 85mm focal, it was too short for me after more then 10yrs with a 135mm (EF 135 L before, then the Sigma). As the 40 Art was the best middle ground between 35 and 50, here I ended up buying the Sigma 105 1.4 Art (a beauty...and a beast, of size and weight!) which today feels perfect indoor, it's long enough for my tastes without getting so cropped in like the 135mm.
I also have the RF 16 2.8 STM, which has basically the same role of the 50 STM, it sits almost unused in my bag, and gets used only in VERY crammed situations, where I have no other choice. I'm absolutely not not a wideangle guy, my dream wedding lens is the Tamron 35-150, I rarely shoot anything under 35, and that's why I immediately swapped my EF 24-70 2.8 L II with the RF 28-70 2.8 as soon as it was in the shops, I gladly traded the 24mm focal for a lighter lens and no adapter needed. So I'm not spending any real money on a 14-35 or 15-35, or even any old 17-40 or 16-35, I almost never use it (less then 100 pics a year...which is still 100 pics more then what the 50 STM gets!), so cheap yes, but especially super light and compact is the key there.