Go to the official web page of any airliner that isn't the 737 and you will see passenger comfort as an attribute of the plane... But Boeing doesn't have anything good to say about the comfort of the 737.
For one thing, the 737-class plane is at a local minimum of seat width. If you make the plane just a little smaller, you can fit five across and get a little more width in economy. Airlines can't do anything to take that away. (And don't need to, because A220 class planes have lower cost per seat mile)
Also thanks to the limitations of 1967 aircraft design, the 737 has a circular cross section. "Widebody comfort" is really about having more height, not more width.
A220-class planes have better industrial design that fits the human body better -- the 737 is like the old American boat cars that were huge on the outside but small on the inside, the A220 is like the Japanese cars that were small on the outside and big on the inside.
A220-class planes also have improved comfort because of fly-by-wire riding out turbulence and with modern geared turbofans they are an order of magnitude quieter than 737-class planes.
More comfort, not just for the passenger, but for people on the ground who hear them go overhead.
You might find it hard to believe, but if you ride in an A220-class plane you will be a believer. That's why Boeing has tried so hard to make sure you won't get the chance to fly in one.