Seat widths have decreased an inch and a half in the last ten years. Adding two inches would only make them 1/2 inch wider than they were ten years ago.
That’s a very good, practical question. Maybe 3 inches is too much.

Please go back to your source. I would bet you lunch that it will say something like average seat width at the major carriers has decreased over the the last 10 years by your 1–1/2 inches. One of the trends in aviation is to use smaller planes for less popular routes. This allows carriers to keep flights fuller, offer more frequent flights or maintain less popular routes. This absolutely increases the profits of airlines and benefits consumers who would otherwise have to drive to more distant airports.