""good" isn't a natural property."
That's where you're fundamentally wrong.
You can't disprove something by defining it it to be non-existent. The term "good" very much describes something real (and natural), otherwise we wouldn't be able to think of it.
Put simply its just the act of fulfilling ourselves, and our purpose. We have a vague notion of good actually is, and are mislead to believe that it doesn't exist (as in your case), for precisely the very reason that we aren't perfect at getting what we need. We get what we want, or what we think we want...which is not necessarily that which is full-filling.
As such, we all have a drive to do what is good, in that we all have a drive to lead full-filing lives. Ethics is the problem of actually leading such lives, not the magical creator of some hypothetical property.
Perhaps you should read (or re-read more carefully) the A Human's Guide to Words sequence.
The term good, by your description, describes something real and natural. Again by your description, X being "real and natural" is required for being able to think of X.
How does any of this reject the statement that "There is no point in eventspace that has the natural 'Good' property"? (which I infer to be the intended meaning of the statement you can "fundamentally wrong")
That some event, decision, action, thing, X is "good"...
I have become convinced that problems of this kind are the number one problem humanity has. I'm also pretty sure that most people here, no matter how much they've been reading about signaling, still fail to appreciate the magnitude of the problem.
Here are two major screw-ups and one narrowly averted screw-up that I've been guilty of. See if you can find the pattern.
It may not be immediately obvious, but all three examples have something in common. In each case, I thought I was working for a particular goal (become capable of doing useful Singularity work, advance the cause of a political party, do useful Singularity work). But as soon as I set that goal, my brain automatically and invisibly re-interpreted it as the goal of doing something that gave the impression of doing prestigious work for a cause (spending all my waking time working, being the spokesman of a political party, writing papers or doing something else few others could do). "Prestigious work" could also be translated as "work that really convinces others that you are doing something valuable for a cause".
We run on corrupted hardware: our minds are composed of many modules, and the modules that evolved to make us seem impressive and gather allies are also evolved to subvert the ones holding our conscious beliefs. Even when we believe that we are working on something that may ultimately determine the fate of humanity, our signaling modules may hijack our goals so as to optimize for persuading outsiders that we are working on the goal, instead of optimizing for achieving the goal!
You can see this all the time, everywhere:
There's an additional caveat to be aware of: it is actually possible to fall prey to this problem while purposefully attempting to avoid it. You might realize that you have a tendency to only want to do particularly prestigeful work for a cause... so you decide to only do the least prestigeful work available, in order to prove that you are the kind of person who doesn't care about the prestige of the task! You are still optimizing your actions on the basis of expected prestige and being able to tell yourself and outsiders an impressive story, not on the basis of your marginal impact.