I think Nikon lost the #1 sales in unit volume probably around 1992, but I'm not a real sales follower, so I could easily be wrong on that end date. Nippon Kogaku (Nikon) was #1 in 35mm "system camera" sales for a long time, from 1959 to at least the mid-1980's, but then again, there really were only two true "system" 35mm SLRS. Yes, as the market leader, Canon does not need to work harder...they have let their sensor manufacturing fall into horrible decline, and they keep churning out cameras with substandard sensor performance since 2007 in the pro market, and since 2009 in the consumer and prosumer segments. Since they are the leader in sales, and since consumer and mid-level cameras make up the huuuuuuuge preponderance of unit sales, they really do not have any incentive to make better sensors. Same thing as McDonald's...high volume, low prices, constant advertising in print and TV media, keeps the "machine" rolling along as the top dog in the low-end food market.
Sales volume numbers/rankings tend to favor lower-cost, cheaper-to-make products. I guess since Hyundai sells more cars than BMW or Mercedes, it logically follows that Hyundai is the better automobile, and has better tech and better performance. Right? Walmart is America's #1 retailer, and is filled with great stuff! Fine furnishings, amazing clothes, top-shelf electronics. Right? The best sh*+ is at Walmart!
Canon's high-megapixel, HIGH-noise 50MP sensor is a joke compared against the new 42-MP Sony sensor. Canon is just terribly behind, but they are McDonald's, and that McDouble at $1.39 is one hell of a lunch item.