I tend to agree with that thesis, and I think a better series of arguments can be made to support it, but here are two arguments for each side:
Without Effective Leadership:
1) Personal Health of Hitler/Assassination Attempt - By the time of the Allied invasions on D-Day, Hitler's mental health (or what was left of it) was shaky at best. Demoralized and depressed by battlefield defeats since Stalingrad a year earlier, he became increasingly reactionary and irrational when issuing orders to his commanders, and increasingly paranoid about those in his own government. The assassination attempt on July 20, 1944 was very nearly successful, and left Hitler partially deaf and with a tremor.
2) Isolation of the German government/Allied bombing - As Allied bombing raids reduced every major German city to rubble, communication, transportation and security became increasingly difficult for Hitler and his general staff. This eventually drove him into the bunker below Berlin, where he was told what he wanted to hear rather than the stark reality facing his country. From there until the end of the war the German SS and Gestapo effectively managed the country while the Wehrmacht fought the war.
With Effective Leadership:
1) Hitler's subordinates managed the country well in spite of Hitler - Those Nazis and commanders who worked under Hitler were often in better mental and physical shape than their Fuhrer, and were intensely motivated by the defense of the Fatherland, and so obeyed Hitler's orders while tweaking them to make more practical and military sense.
2) Climate of Fear/Security Apparatus - Most of Hitler's opponents and potential opponents had been arrested, killed or sent to concentration camps by 1944, and the Gestapo secret police and the SS ruled by fear with an iron fist. The conspirators in the July 20th plot were hanged with piano wire and it was broadcast over the radio. No effective resistance to Hitler took place after that, and while it was a losing cause, Hitler still managed to stay alive and in charge to almost the very end.