Issue one - our CB's do not have the size for press coverage. If we attempt this, there is a very good chance that they will get beaten deep.
Issue two, our safeties do not have the speed to recover deep, bringing them in closer to the LOS (and at times blitzing one of them) would risk a deep strike to a open WR.
At this stage in the team's development, as difficult as it may be for fans to accept, the philosophy is for the opponent to be required to execute a long, multiple play drive to score. I realize that it sounds weak basing a defense on waiting for an opponent's mistake but unfortunately this is where we are right now.
I don't see how the staff can sell the team on a gambling, high risk, high reward philosophy in mid-late October when an entirely different philosophy was preached until now. Once a more aggressive defense can (confidently) be run without a very high risk, we will tighten up our zone (also, another offseason with Balis for the likes of Ormsby, Carrezola, Stapleton, etc. will help greatly) and improve our pass rush.
I do have some background on this (being a lifelong NY Giants fan) as we (the Giants) had a horrific offense, a weak secondary but a ridiculous amount of talent in our defensive front seven in the mid to late 1970's (think the team with Pisarcik's fumble). When we changed regimes (and brought in Bill Parcells as defensive coordinator) we no longer blitzed, we no longer had defensive linemen shooting the gap (which led to Jack Gregory and John Mendenhall eventually moving on in trades to rid the team of disgruntled players) and our secondary played a deep shell in coverage. The defense went from one of the better units in the NFL to a couple of seasons where while (with a couple of exceptions) not giving up an extraordinary amount of points, was giving up more yards than most teams. Once the defense added players, the secondary began utilizing tighter coverage, the defense would blitz on opportune times and the defense became one of the best in the NFL. For the record, Parcells taught Al Groh who taught Bob Diaco.