Let's assume that we have full nodes in the amount that is sufficient for the full operation of all users and the system. Let it be 5 thousand.
After that, we will increase the number to 10 thousand. What will we get? The answer I came up with after much thought: NOTHING.
You are claiming that there there is a benefit of going from 4999 to 5000, but there is no benefit of going from 5000 to 5001. Why does the benefit suddenly drop to 0?
Why is "sufficient" enough? There are plenty analogies of how sufficient is good, but more is still better. Without a concrete measure of the value of a node and a way to determine how many are sufficient, it is impossible to say with any certainty that any particular number is sufficient or that any additional nodes provide no value.