There are many reasons for the IQ, I wouldn’t say the top end recent L glass is behind the competition in sharpness, but certainly lenses like the 50L or 85L isn’t the sharpest out there, they are more about the rendering, more pleasing look rather than sharp enough to cut through a log.
I have some very sharp images from the 5D4, but I do find the Sony sharper, but on occasions the sharpness can be a negative. This is getting very picky but may be I am used to Canon, the look of the files is a bit more pleasing. Sometimes on the Sony, I zoom in and see all the pores and wrinkle and think this can be unflattering to the subject, not something I notice much on the Canon before. I guess you can dial down the clarity in post. There really is a lot in lens design in just pure sharpness, I find the Canon walks the fine line between sharp and flattering and lean more into flattering, whereas Sigma goes for the sharpness by the throat. Sony lenses in my limited experience it depends, the 35ZA isn’t as sharp as the 35L, but bokeh is nice. The 50ZA is SHARP and looks great. 85/1.8 is like a razor but missing the soul. The 85/1.4 GM is better but Canon 85L renders that bit better.
This is all very very very picky though because on the web they all look fantastic. I have shot some good images with the Canon 50/1.4 which by all accounts, one of the lesser lenses.
If you are going to PURE sharpness then Sony A7R3 with Sigma lenses, that’s probably the way to go if you want ultimate resolution in sensor and lens, focusing and lack of AA filter, throw in pixel shift too and I can’t see any combo out there to beat it.