I can understand why you feel cheated, but there is no real advancement from the 737-300 in terms of raw figures as 125 seats is becoming too small for later periods on 2.5tns/hr. Many operators are replacing them with 737-800s, especially for the longer routes where they become more economical.
SWA use them so successfully because they have more seats in their config and are mostly using them on routes around 400nm, with exceptions. In fact I am surprised that Boeing even included the 737-7 in the 737max lineup, if it was not for SWA. Then again I was also expecting them to push for a 199/200 maximum seat and a 757-200 size aircraft rather than keep NG fuselage lengths but they got lazy and ignored the stakeholders to go for a quick certification.