There was from a musically formal standpoint no ‘collaborative effort’ , as such, between the four members of the Beatles band,. from 1962-1970. Each and every one of the Beatles' songs and albums were laboriously and minutely musically written, arranged and/or orchestrated, as the case might be, by George Martin and it was George Martin himself who wrote the songs, not Lennon and/or McCartney who didn’t possess the songwriting talents or skills to write their out of a paper bag, separately or as a team. Paul McCartney to this very day still possesses no such talent.
The most important and fundamental error in your comment is in aknowledging George Martin’s influence on the Beatles but simultaneously failing to acknowledge, recognize or even mention George Martin's total absence or lack of influence, precisely after 1970 during Lennon and McCartney's dismal solo songwriting careers, because essentially therein lies the definitive stylistic musical key or elements that fundamentally and categorically reveals a musicological fact: that each and every song written by Lennon or McCartney individually or as a team, both before 1962 and after 1970, is from a musically harmonic, melodic, literary/textual, formal, analytical and musicological point of view structurally different and mutually exclusive from the hit songs that George Martin wrote and produced for the Geatles during their ‘glory’ years from 1962 to 1970.
The different song catalogues in question, ie., Lennon and/or McCartney's pre-1962 and post-1970 song and album catalogues, on the one hand, and the George Martin Beatles’ song and album catalogue that contains the hit songs that he wrote and produced for the groug from 1962-1970, are fundamentally and categorically different and mutually exclusive from a musicological, ie., artistic and stylistic and form analytical point of view. Lennon and McCartney's trite songs and albums bear their simplistic and unsophisticated stylistic musical fingerprints or imprint, just as George Martin's songs and albums necessarily and unavoidably bear his own stylistic musical imprint, and these stylistic musical imprints or fingerprints, just like our own physical fingerprints, are categorically untransferrable from one person to another.
Given that you make mention of the subsidiary detail, George Martin could indeed have performed the same ‘function’, as you put it, for any band, however, he signed the Beatles under contract not any other band, and it was precisely for considering them to be the ideal vehicle for capturing the lucrative teenage demographic, in Britain and especially in the USA. George Martin made his immense personal fortune along with that of the Beatles precisely by implementing his marketing strategy with phenomenal and unprecedented success from 1962 to 1970, inclusive. What conceivable or rational reason would he, or anyone in his positon, possibly have for seeking out any other band?...Your observations, your rationale and your arguments are pointless, of course.