If you have more engines you have more capital and maintenance cost and more compexity in coping with technical faults in service. However the world has changed with a dramatic increase in fuel price, so we should expect to see some significant change in engineering solutions, yet 2021's 737 looks almost identical to one from 30 years ago. That doesn't seem a proportionate response to me. Someone should be coming up with more radical ideas. I know the manufacturers have their Ceeses sketching interesting concepts but they don't seem to be moving towards realizing these ideas.