Defense-in-depth was based upon the assumption that the outer frontiers could not be made impenetrable, and, since they would eventually be penetrated, a small reserve army that traveled to the point of attack would be insufficient should an attacking enemy penetrate more than one frontier area at one time. Such an invasion could only be stopped if the frontier defense was realigned with strong forts that were built in a deep band, that is, staggered not only along the front lines, but behind one another to form a "running" defense, with the strong mobile army to respond (by region) to any attack.
As also was previously discussed (last paragraph of answer #1), the ultimate conclusion of the defense-in-depth was the deterioration of the Roman Infantry. The limitanei (frontier troops), were no longer expected to defeat the enemy, and before long, they no longer wished to even engage the enemy. All of the training and expectations of actually defeating the enemy fell on the shoulders of the mobile army, thus, reducing Rome's overall combat manpower effectiveness.
This is not to say that these effects were felt instantaneously upon the Empire, in fact, the defeats in Persia and at Adrianople were more from leadership failure than from lack of training or fighting spirit--but, nonetheless, the seeds had been planted from the conception of the defense-in-depth strategy.
The limitanei actually maintained some resemblance to their predecessors although up into the fifth century, at which point, they dissolved into a peasant militia, mostly due to the 'federation' of barbarian forces. So, if the limitanei were still somewhat effective, how did the defense-in-depth help cause the fall of the Roman Empire? Simply, it was not the overall concept of the strategy that failed, but rather, the execution of the strategy. For example, the drastic...