I doubt that Disney really cared that much about ENDGAME breaking AVATAR's record, until they saw that it was something that fandom cared about. Otherwise, they would have had a bigger surprise for the expansion release. They just saw that it had become an obsession for some people, so that presented the opportunity to make more money by feeding that hunger with a few added end credits and expanding to more theatres again. Can't argue with commercial enterprises trying to make more profit--that's what they do.