I recall that John Lennon once said that the Beatles were expected to top themselves with every record. This clearly wasn't happening after Pepper (imho).
George Martin said that that the guys never recycled and never repeated themselves.
Even with the early songs - although all of them were love songs - they tried to do things differently. As Paul said, She Loves You was written in a form of addressing to someone,
P. S. I Love You is in a form of love letter etc.
Not only that they wanted to top themselves with every record - John & Paul were constantly competing with each other.
Paul was making Penny Lane - John's response was Strawberry Fields. Both song are about their childhood memories - but with a totally different approach, music & lyrics.
The two of them made 'A Day In The Life', which is considered as one of their best.
'Rubber Soul' is different than 'Revolver', 'Sgt. Pepper' is different than 'Magical Mystery Tour'.
'The White Album' has 30 songs (and anyone can pick 20 of them which he likes the best and put it on one CD to enjoy.)
Isn't the 'Abbey Road' another peak, one of the best albums, with that wonderful B-side medley, and gems like Something, Octopus's Garden, Here Comes The Sun, The End,
Golden Slumbers, Come Together...?
But it wasn't the end, 'Let It Be' was their final released album - and again, they had the concept to make a 'live' album, returning to their roots.
(Eventually, it came out as 'half live' with song like Get Back, One After 909 and the orchestrated ones, which I like better - Across The Universe & The Long And Winding Road.)
I disagree that the band wasn't making great music after 'Sgt. Pepper.'
And what about magnificent single Hey Jude (1968.), John's Don't Let Me Down and super-crazy You Know My Name (Look Up The Number)?