Over exposing your photos will destroy the natural lighting condition like nothing else will and its one of the hardest things to fix in post.
I have no idea what you mean by "destroy the natural lighting condition".
However, I will agree that over-exposing or over-brightening your photos with too high an ISO is one of the hardest things to fix in post. In this context, "over-exposing" means exposing such that pixels that would, at a lower exposure, have contained data you wished to keep were instead blown. Over brightening means setting the ISO so high that pixels that, at a lower ISO, would have shown detail you wish to keep are instead blown. Note that in this context, whether a photo is over-exposed or over-brightened has nothing to do with whether the exposure and brightening from ISO results in a picture with a different brightness than that which the camera's meter suggested. A picture taken below the metered exposure value may be over-exposed.
Something that is just as hard to fix in post is under-exposure. In this context, under-exposure means exposing or failing to add gain such that pixels in shadow areas that would have contained shadow detail you wished to capture at a higher exposure or ISO instead do not have differentiated values.
When a single photograph has both areas that are under-exposed and areas that are over-exposed, then the scene has more DR than the camera. In such a case the photographer either has to choose which desired detail will remain uncaptured, or has to resort to multiple-exposure HDR techniques.
When a single photograph has areas that are over-exposed and no areas that are less than a third of a stop from being underexposed, that image is overexposed. When a single photograph has areas that are under-exposed and no areas that are less than a third of a stop from being over-exposed, the image is under-exposed.
Far safer to underexpose if anything and correct your exposure values in post processing than anything else.
If you underexpose, you create two problems: you lose detail in shadow areas and you get more shot noise than is necessary. If you overexpose, you get one problem: you lose detail in highlights.
You can adjust your exposure values well enough in post processing even if your original light reading is out 1 to 2 EV with none/negligible image degradation.
You cannot adjust exposure values in post. You can only adjust brightness. Adjusting brightness does not create any new data, it just shifts data to different parts of the bit-array available to a pixel. In the process of doing such adjustments, it is possible to lose data.
You can bring it back up to what it would have otherwise been at 0 in any decent RAW processing software.
No RAW processing software can take multiple pixels which all have the same data value and create the same differentiation between them that would have been present if either the exposure or the ISO had been higher.
As far as not adding read noise with the same exposure values as you increase ISO I'll call you out on that one also. At least that's not how it tends to work in reality. There is no such thing as a free lunch when it comes to ISO.
I suspect you don't know how ISO works. If you want to "call me on that", you're going to have to show how and why increasing ISO adds read noise. Actual real-world data would be helpful. Here's a hint for you. Data such as DXO's SNR 18% graphs doesn't show increasing read noise with increasing ISO, It shows decreasing SNR from a decrease in signal greater than the corresponding decrease in shot noise.
If there was such a thing with a free lunch with ISO, we wouldn't have this constant debate about signal/read noise and its characteristics in relation to the effect it has on images from different sensors and no one would bother going for a larger sensor to reduce ISO related noise issues.
Bigger sensors don't reduce ISO-related noise issues. Bigger sensors reduce shot noise. They do this by capturing more light for a given exposure.
I'm sorry but that discussion speaks for itself.
I'm afraid you don't understand that discussion.
If it was perfect and the effect was in relation purely to other factors we wouldn't even bother looking into the matter of reviewing photos at various ISO settings and everyone would be happy to walk around shooting photos at ISO6400, but they're not...
Correlation is not causation. There is a correlation between ISO and exposure. When the exposure is low, people use high ISO's to get desired image brightness. The low exposure causes both the lower SNR and the need for a higher ISO.
We accept that using more ISO creates noise
No we don't. Only people who don't understand what is going on "accept" such a thing.
no matter whether the settings other than ISO are the same, or otherwise and the fact that using more ISO adds noise to an image
It isn't a fact.
regardless and if I can see it myself taking three shots at the same aperture settings and shutter values with 3 different ISO settings your what they call in simple terms not correct.
Except you cannot see that. All you can see is a difference in brightness.
