Opening day MK attractions that remain, 14 : CBJ, Dumbo, Frontierland Shootin' arcade, HoP, HM, It's a SW, JC, Mad Tea Party, PP, Carrousel, Swiss Family Treehouse, T Speedway, Main St Vehicles, and the Tiki Room.
Attractions that opened in 1971: LS Riverboat
Attractions that opened 1972-1974: Tom Sawyer Island, Pirates
MK now has 39 attractions. So only 35% of the current attractions are original. if I expand that out to the first three years, still only 17/39 or 44% of current attractions date to the first three years the park was open.
CBJ- though largely similar, it underwent a significant update in 2012.It is 5 minutes shorter, with new outfits and props. (One thing removed was this dated fat joke, "That's a mighty big song, Trixie!" with response, "That sure ain't all that's big!")
Dumbo - major revision
HoP - Multiple revisions since Nixon was president in 1971, and the Saturn V rocket was featured, though it mostly remained original until 1993.
HM - gone are the spiders, and the queue is pretty different.
It's SW - no longer sponsored by Mattel, and was changed quite a bit in 2009
JC -the original JC was more serious, not full of jokes as it is now.
PP - queue got significant updates recently, ride has had some small ones over the years
SFR - ? not sure I think it had some updates over the years...
T Speedway- One could say it didn't change much over the years, except it just closed and the track is going to be changed for the 4th time. In the past the track was shortened, and name changed in 1994 and 2008.
Tiki room - was quite different 1998 - 2011, until it changed back to almost original show
So of the original 14, I'd say only about 5 have not significantly changed since the park opened: the Shootin' Arcade (maybe some changes? not sure), Mad tea Party, Carrousel (some changes, but minor), SFR, and Main St Vehicles. I wouldn't call any of those attractions blockbusters. most folks walk right past the arcade and Main St Vehicles.
mind, I realize, as palces go, most WD change has been very gradual, and much of the vibe of the 1970's has remained, but the park has also changed a good deal since the 1970's.
I think we just don't see much of it, because it has been gradual. WDW has done a great job of doing that. I think we also tend to forget some kinds of changes, because we want MK to be the same. Nostalgia kinda goes both ways.