The Decline of Intelligence in Modern Humans @L'éléphant Apologies. I was lazy that day and didn't bother to read the OP
:D
What you have posed is a possibility but we can argue the opposite too. How can we measure this realistically? I don't think we can as there are far to many factors involved and many cognitive abilities are not exactly well understood by any means.
For the sake of arguing against I could suggest that agriculture allowed us to free up our time and work together in groups more easily (specialisation). Of course there are counter argument to this too as there is reasonable evidence to suggest that human collaborated on a pretty large communal scale prior to the full blown advent of sedentary living and/or agriculture.
I would also argue that 'intelligence' for humans is something that expands due to better lines of communication and interaction (something that has become increasingly prominent in the modern era), but again I could offer up an opposing view that may partially agree with this overall, yet question whether or not there is an optimal amount of 'communication and interaction' and that too much of this would actually start to reverse progress.
Finally, one more part of this puzzle ... education and pedagogy at large. We seem to have struggled to adapt our education facilities to changing times of late. The industrial revolution had, in my view, a partly detrimental effect upon education standards as it copied and pasted the 'factory' method by viewing schools as factories for employable citizens. In our times now I think this has mostly been realised, yet not by any means addressed fully. Now we have a weird social landscape in which there seems to be a growing inclination to throw the baby out with the bathwater in terms of educational syllabuses and the general view of pedagogy at every stage from primary school to university (although the latter is a little more robust imo).
Less intelligent? I don't think so but it could be true; in terms of genetic predisposition. I think it is more about our educational systems lagging behind population growth, political shifts and trends, and increasing technological advancements that we've had little time to fully utilise.