well, mccartney, while still hugely popular, is not among the type of current star that the media, in their zeal to sell-sell-sell (that is half their job: always has been, alway will be) would turn to often, especially thoughtfully.
so we have only has thoughts to rely on, and those are best exemplified by what he puts on the albums/into the songs.
i've often thought that nearly every album from T.O.W. on is alot more mature and thoughtful than most that came before (not that he was/is as much of a lightweight as some fans & critics have often made him out to be).
this would seem as no surprise as he would now (then) have been well into his 30's and spending as much time raising kids as raising he...ck.
you are quite right, it has been an amzingly creative, and rather feep period, as life itself takes its toll, on he and his friends/family, he has responded to lifes everpresent march by being more and more thoughtful and reflective.
as to what 'biographers' have missed...well, just about everything of signifigance: they've dwelled almost solely upon the brash and the harsh; whether death, aging or loss.
which, in fairness, is what they do with just about everybody, and, again, almost always have done so.
the biographers are just an extension of the media, and the media, while not perfect, live within the same economic constraints all the rest of us do. if it does not sell, they will, more often than not, not go there.
justin beiber & his girlfriend sell...prince william and princess kate sells...paul mccartney ?
not so much, anymore.
...just another higher reachin' hillman !