Newby's and Chapman's quitting weren't alike in the least and Chapman didn't quit.
Chapman was going to be Ringo Starr until the British army came calling him as one of the very last draftees into their army...
Newby was offered Sutcliffe's job by John Lennon for the second Hamburg tour but he opted to remain in school.
The Beatles easily could have become Lennon, McCartney, Harrison, Newby and Chapman for the remainder of their years.