I've used the Canon EF 50mm f/1.8 II plenty in the past. A lot of folks talk about how fantastic a lens it is. They've even nicknamed it the Plastic Fantastic. That's mostly because it's the first prime lens they've ever used. It's not a great lens. When used wide open it's not even a good lens. It is a fantastic value for the price, particularly when stopped down to f/2.8 or narrower. It's also better than most kit zooms used wide open when the 50/1.8 is stopped down to around f/2.2 or f/2.5, which is still faster than any of those kit zooms.
Normally I'm the first to write answers that advise folks to improve technique and get the most you can out of the gear you have instead of thinking a camera upgrade will solve the photographer's own artistic limits or poor technique. Most issues with blurry pictures are due to poor technique. A better camera or lens doesn't make anyone a better photographer, it just reduces the limits imposed on the photographer by the gear.
But even the latest, greatest, best, and most expensive gear can impose limitations on the most skilled photographers. The reason those photographers are the best is because they know the limits of the tools they are using and are able to work just within the boundaries of those limits. They also understand which of the multiple tools in their bag are most appropriate for a particular shooting situation.
But in this case, honestly, the example on the right is about the best I think one can expect from any of the three versions of Canon EF 50mm f/1.8 lenses when used wide open. It really is that soft when used at f/1.8. Even if the camera were on a sturdy tripod, set to use mirror lockup, and triggered with a remote shutter release, I doubt one could do much, if any, better with any of the three versions of the Canon 50/1.8 when used wide open at f/1.8.
On the other hand, the Sigma 85mm f/1.4 ART is known to be a nicely sharp lens, even wide open at f/1.4. That's why it costs about ten times as much as the Canon 50/1.8.
Even stopped down to, say, f/4 and used on a tripod with remote release and mirror lockup I doubt the EF 50mm f/1.8 could do any better than the Sony α7C/Sigma 85/1.4 shot, other than regarding camera movement. If one looks carefully at the Sony shot, one can see evidence of rotational movement in the 'E' and both 'S' letters at the bottom of the frame. Perhaps it's a combination of camera movement and IBIS correction that counteracted camera movement in most, but not all, of the frame?
Sometimes it isn't all about technique. Some rare times it really is the gear.
You may have heard: Gear doesn't matter.
While very often true, it is only half the truth.
The full truth is: Gear doesn't matter... until it does.
When the gear does matter, it really does matter.