Revera, you are mistaken. From this set of verses we learn what we already know that one may risk his life to go to battle to fight our enemies. But how much of a risk does a King have to take in going to the battle himself? So it maybe teaches us another possible lesson, what is considered too much of a risk for a King in battle. We learn if the King loses his strength due to age like in this case then it is considered too much risk for a King. Alternatively, you may learn nothing out from here since what the Soldiers say is not necessarily Torah, it's that they personally were worried about David, perhaps wrongly and they should have had more faith in G-d. This would not be the first time the public was wrong, as they asked for a King for the wrong reason as you know.
As for comparing this situation to Chaim, it's very weak if this verse talking about a King going to battle weakened is a good comparison. Chaim is taking about the same risk as going to battle according to me. And with regard to the Rav, he also took about the same risk going to battle. Your insane remark that he was wrong since he got too weak is an insult to the great Rav. His strength did not leave him, in fact I think he was even stronger and wiser as he got older and he was relatively young in years when he was taken. Remember that the Rav was shot and had he been younger in years and this happened, he still would have died since you basically can't survive a bullet to the head. The Kings of Israel fought many risky battles like the one that Killed King Shaul, but the Torah requires them to fight such battles to save Israel and trust in Hashem.