I think the obvious answer to
the whole thing is that Brian Wilson built a mountain he couldn't climb and
he eventually knew  it, but, since the stakes were so high, he kept
recording stuff that he thought would tie together all these elements and
finally got to a point where the record had mutated into something he could
never shape together, and at the same time, he couldn't deal with the
professional/family pressures that would have broken anyone his age with his
responsibility.