Balance is key....if the park spends $120 on a new E-ticket, yeah....that will draw people in, but what will keep them there? After they ride the 1 new attraction.....if everything else is old, dated, or non-relevant (i.e. MGM) what keeps people in the park, and/or coming back.
Now, look at the MK....the park has not opened a huge new attraction since 1992 with Splash Mountain.....but what they have done is open a ton of filler....even in the last few years. When the MK finally has a new MAJOR E-ticket built, there is plenty of other attractions to keep people occupied.
On the other hand...MGM....opened an E-ticket show in 2005.....E-ticket ride in 1999, and E-ticket ride in 1994....yet none of the other core attractions have changed much since opening....it is still the same shows since 1991, the same core rides since 89/90.....the park has NO balance. MGM could be due for another E-ticket, as its attendance is still off majorly from the 90's peaks, but what good would it do....IoA proves that a park with a ton of E-tickets, but little in the way of core attractions, is not going to be a major success. While MGM could benefit from a major E-ticket...opening it without the correct support attractions in place would be a waste.