I think the only sense in which God is “the only plausible ultimate reason for being moral” is that God provides universal guaranteed enforcement. In other words, God lets you make this claim in any situation: “if you want to be rewarded instead of punished, then you should behave morally”
Other interpretations (e.g. the Rawlsian one I mentioned earlier) say that morality is a feature of rationality itself. The Rawlsian claim would be “if you want to be a member of the rational creature club, then you should behave morally.”
Nope.
The reason is because God provides ultimate purpose and meaning in terms of intentional teleology that matter by itself simply cannot.
Morality is an intentional enterprise which positively requires intentionality built into the fabric of existence.
No one said anything about “guaranteed enforcement.” In fact, that requirement runs contrary to the notion of obligation and responsibility wherein moral agents autonomously carry out moral actions.
Now, there may be the incidental fact that failed moral agents make themselves vulnerable to certain consequences, but that may be a simple matter of what it means to be a failed moral agent.
In fact, it could be agued that moral agents who act morally merely because they are compelled or forced to do so by fear of consequent punishment are not acting as moral agents in true any sense of the word. They are, if not failed moral agents, definitely compromised moral agents. “Enforcement,” in this sense is more like having training wheels on a bicycle. It keeps the agent from harming themselves or others, but doesn’t permit them to ride on their own as autonomous moral agents until the “training” comes off.
Guaranteed enforcement merely protects the overall moral climate and the individual from harming themselves and others, but doesn’t positively or negatively affect the formation of any true moral agents. In a sense, it acts as a scaffold to keep the agent “upright.”
Reward and punishment works for training any sentient being, but doesn’t bring about the development of uniquely moral beings. At best, it is a neutral factor which is why, I would submit, it is in place.