That's completely false - it really couldn't be further from the truth. Not only do we still have quite a few low-post players in the league, but ONE "dominant big man" does not explain the fouls/free throw discrepancy - especially since we do have a dominant big man in Cousins - who draws more fouls than Duncan ever has in his career. Not only that, but the percentage of inside shots hasn't decreased over the years; teams are just cutting down on low percentage shots (midrange/long 2s) and replaced them with more high percentage shots (in the paint) and higher points per shots (3-pointers).
Teams aren't taking more jump shots today though.
Here's the thing, 3-pt shot attempts have increased but not OVERALL jumpshots. As I mentioned before a certain portion of those midrange/long 2s have been replaced with players stepping out a few feet further behind the arc.
That's not to say that the increased ratio of 3-pt shots to total shots hasn't had an effect at all (it's really had an impact on spacing more than anything else, which ties in with my next point), but it's certainly not one of the main reasons. I'd put this mostly down to offensive and defensive schemes being better today, and the influx of data and information teams now have to adjust/counter to find ways to score and stop scoring.