At this level, " better" is a relative term. The OP may have been hasty in labeling the 24-105 as amateurish but it is not as "good" as the 24-70II. Unfortunately, small improvements carry a hefty price tag in premium lenses.
yes, exactly - there is a diminishing return as you go up the scale. I probably would have kept the 24-70v2 anyway, but I was shocked at the results around 45-50mm - they were worse than the 24-105. I might have had a misaligned copy, but the reviews I have seen agree that the midmark isn't "stellar".