I'm sorry, but what are you looking at, because the computed MTF curve for the Canon 35mm f1.8 certainly is NOT better than the actually measured (with 10 copies) MTF from the Sony 20mm f1.8 by Roger. The 10mm san/tag lines from the Sony are substantially better than the Canon, they also show no separation out to the full frame corner while the Canon has substantial separation (hence, the awful aberrations you see in the example images as posted in the various threads linked to the Canon user reports). The 30mm tan/sag lines are sharper for the 20mm before falling off to similar levels, the 20mm lines are help show the lack of distortion and again have virtually no separation.
FWIW there's really no point in comparing MTF charts from such different focal lengths, especially ones provided from a manufacturer which are computed and those from a private tester which are "real". Based on reports (and my own personal experience with the Sony 20mm) it's quite apparent that the Canon 35mm can't be used wide open unless you are ok with pretty bad coma and LoCA, the 20mm on the other hand can be used wide open with virtually no coma and LoCA.
To the other comments regarding Sony lenses, I have used quite a few now for astro, the Zeiss 50mm f1.4ZA, the Voigtlander 50mm APO, Voigtland 65mm APO, Voigtlander 110mm APO, Voigtlander 21mm Nokton, Voigtlander 40mm Nokton, Sony 24mm GM, Sony 20mm G, Sigma Art 14mm (e-mount), Sigma Art 35mm, and a few others. I would say the ones that come closest to "perfect stars to the edge" are the Voigt 50mm APO (this is the best 50mm currently available except maybe the Leica APO) and the 20mm G. The Voigt 50mm APO is a work of art, one of the few lenses capable of resolving the high resolution A7RIV sensor, no coma, very very little LoCA. The 20mm G is probably the best lens in the 16 to 24mm range, I've also used the Nikon 20mm f1.8S which is very close, but the Nikon has worse distortion and a baked in lens profile that sucks. The Sony 20mm G has very little distortion, has slight amounts of coma, but no LoCA. The 24mm GM is a little bit sharper, but has slight coma and visible LoCA, it needs to be stopped down to f2.2/f2.5 IMO. The rest of the lenses that I've used have varying degrees of coma/LoCA or sharpness fall off. The Voigt 65mm APO is also fantastic, but quite a bit heftier than the 50mm and I think the 50mm is sharper. I've been using the 110mm APO on my ASI2600mc, I'm working on a high resolution mosaic of the MW this summer with that combo. Voigtlander is going to be releasing a 35mm APO which should also be fantastic, possibly the best 35mm available based on its MTF. I tend to mostly use focal lengths wider than 35mm for time lapse, so in those instances I'm doing untracked single exposures, focal lengths 35mm and longer I use for tracking mount based nightscapes and stuff of that nature. I've got quite a few time lapse videos with my A7s and 20mm G (plus a few other lenses) here:
Lots of tracked nightscape shots with various lenses here: