While I acknowledge that some of the responsibility for the poor back play has to sit with the coaching team, what we're seeing more often than not is basic poor handling. Passes behind the man, passes at head/ankle height, poor timing from the runner/passer, failing to catch the ball etc.
I coach under 9s these basic skills on a Sunday morning. Simple drills to build an understanding among relative novices with the concentration level you might expect from a 9 year old. At a very basic level, the kids can make it work most of the time, and they will only get better and faster as they get older. Now I've never had to opportunity to see how elite, professional rugby players train, but surely the coaches aren't still drumming these basics into them, running the same drills they were doing back when they were under 9s, are they? This should be a given, and the coaching team can't be held accountable for basic, individual errors. Whatever the coaches are doing with them through the week, mis-directing or mis-timing passes and failing to catch the ball are not among them. Maybe there's a lack of clarity/understanding in the messages from the coaches that is muddling thinking and leading to basic errors, but ultimately poor execution is down to the individual.