I think that the nature of the question has to be analyzed. From the question, the implication is that Paul's "downfall" was the result of, or at least impacted by, those around him. It is as if he was trying to repel what was around him. I think that Remarque's condition of war is so powerful because it was "equal opportunity." It impacted everyone. The condition of war that awaited the soldiers, particularly young ones, was so stark that all of them were impacted in different ways. It was not as if Paul was able to maintain a condition that was withered away through the negativity of others. Rather, it was something that impacted all of them in an equally brutal manner:
To-day we would pass through the scenes of our youth like travellers. We are burnt up by hard facts; like tradesmen we understand distinctions, and like butchers, necessities. We are no longer untroubled--we are indifferent. We might exist there; but should we really live there? We are forlorn like children, and experienced like old men, we are crude and sorrowful and superficial--I believe we are lost.
When Paul speaks of "being lost," it is not a reflection of being influenced by others. Rather, it is the result of the war that is raging around them, a war where there is little in way of hope and restoration. It is a war where suffering is limitless and pain is abundant. Living a "forlorn" life is applicable to all soldiers. Paul is not fighting a fight of positivity against negativity. Rather, Paul's experiences are reflective of a reality that no one had anticipated.
The second part of the question would be whether or not Paul experienced a "down fall" in the traditional sense. When the term "down fall" is invoked, it traditionally means a fall from grace. Sometimes, it means a descent triggered at the hands of an individual. Paul's "down fall" is the reality that besieged all of the soldiers who fought in World War I:
How long has it been? Weeks--months--years? Only days. We see time pass in the colourless faces of the dying, we cram food into us, we run, we throw, we shoot, we kill, we lie about, we are feeble and spent, and nothing supports us but the knowledge that there are still feebler, still more spent, still more helpless ones there who, with staring eyes, look upon us as gods that escape death many times.
Paul's descent is the result of a futile war experience where the summative function is one of being "feeble and spent." The people around Paul do not contribute to this condition. It is the reality of war. The manner in which Remarque depicts war is so far reaching and so expansive that soldiers cannot escape from it. Time passes and there is no other way for the soldier to evade this condition except for death. It is for this reason that Paul's death is described as one where he "had an expression of calm, as though almost glad the end had come." Paul's downfall is not shown to be something as the result of individual causation. Rather, it is the wide ranging reality that governed all soldiers, reflecting the horror and brutal nature of modern war.