The MK wasn't a half-day park, it had more attractions in 1971 than DAK has twenty years after opening...
We all want to think with rose colored glasses, but this lineup is really sad compared to today and even compared to other park openings...It was never designed to be a full park on opening and the idea of a "full day" park wasn't really a consideration on opening. The park was there as a family draw for the other "resort" features (golf, lounges, dinner shows, tennis, boating, etc...).
When you boil the attractions down to the core, there is not a lot to do...A few spin rides (Dumbo/tea party/carousel), some true classics (Mansion, Peter Pan, Jungle cruise, IASW, 20K, MTWR), a few animatronic shows (Tiki/Bears/MMR/HOP) and some "filler" (Keel boats/Riverboat/stare at the floor THE RIDE!/various versions of cars/shooting gallery).
MK was extremely sparse until the 80s and was not at all what you see today. When you look at that list, there are two, maybe three current "E" ticket rides and they are all dark rides.
When the MK opened, we didn't have BTMR, SM, PotC, Splash, Star Jets, If You Had Wings, CoP, TTA, train stations or circle vision...it was a park you went to with the family to get the kids tired before you went to Broadway at the Top...