the 40 Art which is probably the sharpest lens on the market, period
That, to me is the one that got away.
Well, actually, I sent it away, because I absolutely had no use for it, but no other prime lens has ever impressed me as much as the Sigma 40mm f/1.4 DG HSM Art.
I would have loved keeping the lens. It cost me less than 600€ (a returned item, again) and it's just so, so good...I wanted to keep it as guilty pleasure.
They had several choices -- the 6D / RP sensor, the R sensor
Those sensors (from the 6D Mark II and 5D Mark IV, respectively) were older and had already been used on mirrorless cameras.
The EOS R was released in 2018, the EOS RP was released in 2019, and the EOS R6 and R5 were released in 2020.
I'd never dare to say that a line of shamelessly optically uncorrected lenses with extreme vignetting is 'a great accomplishment'. I don't even think the VCM line is worth of the red ring. At that price tag the least I expect is the lenses to be optically corrected.
The 50 and the 85 are somewhat similar to their predecessors in that regard. It's just the shorter lenses that have a lot of distortion correction via software but, even with such correction, they're delivering higher image quality than their predecessors.
Actually, in terms of vignetting, there are many EF lenses with tons of it as well. Some are even worse than the VCMs.
Review vignetting (peripheral shading) test results with f-stop contours for the Canon EF 24mm f/1.4L II USM Lens. Compare the results from this lens with other lenses.
www.the-digital-picture.com
Review vignetting (peripheral shading) test results with f-stop contours for the Canon EF 35mm f/1.4L II USM Lens. Compare the results from this lens with other lenses.
www.the-digital-picture.com
Review vignetting (peripheral shading) test results with f-stop contours for the Canon EF 50mm f/1.4 USM Lens. Compare the results from this lens with other lenses.
www.the-digital-picture.com
Review vignetting (peripheral shading) test results with f-stop contours for the Canon EF 85mm f/1.4L IS USM Lens. Compare the results from this lens with other lenses.
www.the-digital-picture.com
As you can see, the 85 VCM is the only one that seems slightly worse.
View the image quality delivered by the Canon EF 24mm f/1.4L II USM Lens using ISO 12233 Resolution Chart lab test results. Compare the image quality of this lens with other lenses.
www.the-digital-picture.com
View the image quality delivered by the Canon EF 35mm f/1.4L II USM Lens using ISO 12233 Resolution Chart lab test results. Compare the image quality of this lens with other lenses.
www.the-digital-picture.com
And resolution improvement is there, for the ones with significant distortion.
Then the EF 50mm f/1.2 L is not a lens for you. But just because it does not meet your needs doesn't mean it is not an excellent lens for other use cases.
I know, I went with the Sigma

I no longer own it though, I sold all my remaining EF lenses last year.
Actually the Sigma 50mm Art was the last to go.
I went with the intermediate solution, the 28-70mm f/2. It replaced my 28mm f/1.4 Art, the EF 24-70mm f/2.8 II, and the 50mm f/1.4 Art.
I still like to own a few prime lenses but now I don’t really use them for work, and this 45 will probably suit me in that scenario.
Having said that, way back at the beginning of this discussion I said that the R6 Mark III "matching" the R5 in a way that was reminiscent of the 6D Mark II matching the 5D Mark III was, in both cases, on the spec sheet, but not in terms of build quality.
They R6s don't match the R5s in terms of weather sealing, but they mostly match in terms of overall build quality.
The 6D Mark II is made out of polycarbonate resin with a few parts of magnesium alloy, with its weather sealing relying on tightly assembled plastics - it's mostly a plastic camera (I had it, I upgraded to the R6). Plastic on top, plastic at the bottom, plastic on the sides, plastic doors.
The R6s have pretty much an entirely magnesium alloy structure and feature weather sealing gaskets. The R5s feature
more weather sealing gaskets.
Heck, the originals R6 and R5 are so similar they share the same cage models.
In terms of shutter expectancy:
The 6D Mark II was rated I think at 150k;
The RP is rated at 100k (it had a lower release price);
The R6s are rated at 300k;
The R5s are rated at 500k.
The R6 is nothing like what the 6D cameras were, it went upmarket. The mirrorless transition brought the 6-series much closer to what the 5D were - minus the resolution, that is.
From basic sensors to high performing sensors.
From basic autofocus to the same features at the high end cameras.
Higher framerates, improved durability, improved build quality, 1/4000s shutter speed limit removed, second memory card slot.
And I'm going to ignore video, as I don't care for that.
I don't buy the "love letter" story though, sorry. The R6 was released at almost 1000€ more than the 6D Mark II, here. The camera went upmarket and we paid for it.
To be honest, I've been imagining the R6s in the future will take the place the 5D DSLRs had in the market, with the R5s going to an upper level, above €5000, closer to the concept of the Sony A1. Clearly that's not happening now, but I'd say it may happen within a generation or two.