And we have 4 class airlines like BR and BA. Why do they do this? Do they really make a lot of money this way? We even have KE having a 4 class but Premium First, First, Business and Economy seating.... Now are all these viable?
Four-class service makes BA plenty of money. Not every destinations is four-class.
Miami, a high-yielding destination with plenty of business traffic, gets a 4-class service. Also getting 4-class are other big business traffic destinations such as Toronto, Philadelphia, Boston, Washington, New York City, Newark, San Francisco, Los Angeles, Seattle, Sydney, Johanesburg, Tokyo, and Chicago (maybe some others too).
Denver, an "average" traffic destination, with good yields and a good mix of traffic, get's standard three-class service.
Tampa, a lower yielding destination, still get's three class service, but with Economy, Economy+, and Business, no first class. Orlando and some Caribbean destinations also have this layout.