You're missing my point. Do we even know if the execution of the "fundamentals" has declined over the years? I mean maybe it is in some areas, but not in others? If you went back and looked at the 1980's era, would you see the vast majority of rundowns being executed in the manner suggested by the OP? Would we see a substantial decline in # of successful rundowns in the 2010's? I wouldn't expect it to be honest, but maybe I'm wrong.
Perhaps teams simply don't want players spending a s*** ton of time to master the art of bunting because that's simply not a prioritized skill in today's game? Maybe the fundamental execution of other in game scenarios has increased? Maybe we make that bare handed throw from 3rd to 1st better than we ever did, or we cut off more balls in the gap, or scoop the ball in the dirt at 1st more, or catchers block a higher % of balls in the dirt. Maybe teams have prioritized other aspects of the game which are deemed more relevant and important in 'today's game' over things like bunting or slapping the ball the other way. It just seems like people fall into the "old man yells at clouds" crowd when a player fails to execute one of the aspects of baseball that 'used' to be a lot more common.