MiddlePath:
Fair point. However, serving Hashem for the reward is considered the lowest level of service, probably because it is kinda self-centered. The highest level is doing something only because Hashem wants it done, with no thoughts about yourself at all.
RSRH:
I believe R’ Dessler held something similar.
Middle Path:
“…if, G-d forbid, a Jew questions G-d’s existence or rebels against the Torah, and he was taught to only do good deeds because G-d wants us to, then he has no reason to do good deeds. If he doesn’t believe in G-d, he will have no problem stealing, murdering, etc., because the only thing preventing him from doing such things is that “G-d said not to”.”
If a Jew ch”v denies Hashem’s existence there’s really very little difference if he does mitzvos or aveiros – he’s not getting into Olam Habba either way.
“Wouldn’t it be safer for people not a such a high level of faith, to be taught the lessons and rules of basic moral conduct that they are simply how a mature, responsible human being should act?”
Absolutely not. We should do the mitzvos not because we have independently decided that this is “moral”, but because this is what Hashem said should do. And if He said to do something, obviously it is the right thing to do. If somebody does the mitzvos only because they fit with his “morals”, he is not serving Hashem at all; he serving only his own made-up morals.