I was curious about whether MiB opening so soon after IoA was industry standard practice, so I took a look. EPCOT opened Norway and Maelstrom 10 months before MGM opened and the Wonders of Life Pavilion with its multiple attractions 6 months afterwards. When Animal Kingdom opened, MK opened Buzz Lightyear 6 months later and Pooh the following year, EPCOT opened Test Track less then a year after AK, and MGM debuted Rock N’ Roller Coaster 13 months after AK opened.
As for shows, Fantasmic! premiered at MGM 6 months after AK opened and Illuminations debuted just over a year before MGM.
That’s just a quick glance at rides opening within a year and a half of a new park. The lesson seems to be that, yes, you want new e-ticket entertainment and attractions to fill older parks when a new one opens… and that Disney used to build a whole lot more.
PS: Magic Kingdom didn’t open anything major within a year and a half of EPCOT, but two years earlier, right about where we are now in relation to EU, a little ride named Big Thunder Mountain premiered.