I'll try to give a numerical example of why ETTR is not a flawed concept,
For simplicity, let's suppose that the possible values for the RGB channels go from 0 to 100. This are the linear values as output from the A/D converter.
The raw data for any given image will be values from 0 to 100 in any channel.
Suppose that as a result of a given exposure we have the following ranges:
R= from 1 to 35
G= from 2 to 48
B= from 1 to 25
If I input those values to a Raw converter, I will get a result, which I'll call result A
Now, let's just multiply all values by 2, we get:
R= from 2 to 70
G= from 4 to 96
B= from 2 to 50
After input this values to the same Raw converter, I will get result B
Now, if we consider both sets of data coming from a black box, we could say that the difference between both is 1EV or 1 stop
How to get output A from the second set of data? Two choices:
1) Divide by 2 all values (which will result in the exact first set of data)
2) Reduce exposure by 1EV in the Raw converter
In 1) you will get the exact result A, no question about it.
In 2) you may or may not get the same result than A, since it depends on how and when exposure correction takes place in the Raw converter
Now, going back to the black box, if we didn't know that the second set of data is just the first set multiplied by 2, we could suppose:
a) The difference is due to a 1EV exposure (time or aperture)
b) The difference is due to different ISO (double)
c) The difference is due to ETTR exposure (note that now the range of G = 4 to 96, covering almost all possible values)
Up to this point, Is there anything on the data that will compromise color rendition? NO, nothing.
If we knew the exact proportion of the ETTR exposure related to the "correct" exposure and divide all raw values by that factor, then we will have as a result the same raw values as if we used the "correct" exposure, but with less noise.
So let me change the conditions of the example: The second set of data is a the result of ETTR exposure which correspond to twice the correct exposure, then just divide the second set by 2 and you get the first one.