The physics of the timeline alteration interest me. At least three people survive the change, which surely has to be temporal in nature, given the presence of an aged John Lennon who never formed the Beatles. There is precedent in the literature for this: in Star Trek: First Contact (Braga, Moore & Frakes, 1996) you can avoid a timeline change of you are "caught in the wake of the temporal vortex" when it occurs; that would however not appear to apply in this case. Another option is that cited in Back To The Future Part II (Gale & Zemeckis, 1989) where changes wrought by one time traveller are avoided by two others because they were in the future when it happened; again, this would not fit the facts in question. Oh, sure, if you want to go off the reservation you could mention Frequency (Emmerich & Hoblit, 2000) but that was judged to not have been adequately peer-reviewed and to attempt to apply it in this case would be junk science of the worst sort.
It's also possible that the change was not temporal in nature, but that some wider distortion to the fabric of the universe was to blame but to which some remained unaffected. Consider Night Of The Comet (Eberhardt, 1984) where isolation from external factors enables survival - agin, likely not applicable in this case. Perhaps more appropriate would be The Quiet Earth (Baer, Lawrence, Pillbury & Murphy, 1985) where only those at the point of death survive the ominously-capitalised Effect. The available facts would appear to support this hypothesis. More concerningly though, such an Effect manifested increasingly-severe follow-up effects potentially heralding the collapse of the universe itself. The subsequent discovery that nobody in the Beatlesless universe now remembers Harry Potter could be evidence of this. If so, it presents us with the enticing possibility that not just the world but indeed the very universe of Yesterday (Barth, Curtis & Boyle, 2019) will soon collapse into nothingness.