The "Get Back/Let It Be" sessions were originally designed as a back to basics album project where The Beatles would get back to their rock and roll roots from the more theatrical period they went through in the latter 1960's. These notoriously fell apart, and rather than get back to some classic rock core, the Beatles parted ways and fired cannon shot across each other's bows for the rest of the decade.
The sessions produced a number of takes of various songs, and random jam sessions, and was in the end abandoned and given to a few people to see if they could take the raw material and put it into some workable form that could be released as an album. Phil Specter eventually managed to do this (his theatrics in adding onto the songs went against the original intention, and is what heavily contributed to Paul McCartney finally leaving the group).
But what if no one managed to take the sessions and make them into a releasable album, and "Let It Be/Get Back" remained unreleased and Abbey Road took its place as the last album?