I don’t think you read my post correctly.
My third point was responding specifically to your example of a user wanting to delete all images from an observation; given that this makes observations casual, I fail to see why users should be prevented from doing so in the rare situation that they might choose to do so, regardless of whether the observation has 0 IDs, 1 ID, or multiple IDs.
My first point was not about users wanting to swap out all photos, but rather some portion of them (because they accidentally mixed up organisms and several of the photos had a different species). My experience is that in such cases it is not entirely predictable which photos the observer decides to keep and which ones they put in a new observation. It may happen that the observation has been given an ID of species X by the observer or another user, which correctly applies to some portion of images, but when the observer edits the observation, they only keep the photos of species Y.
Eg: photos 1, 2, 3 and 6 are species X; photos 4, 7, and 8 are species Y. Observation is given an ID of species X. Someone comments that the photos are not all the same species. Observer edits the photos and keeps 4, 7, 8. Observation now has an ID (species X) that does not apply to the photos (species Y). It is irrelevant whether this ID was entered by the observer or someone else.
Your solution of locking one photo would not fix this, because there is no way to automatically determine which ID belongs to which photos.
The result is still a wrong ID on the observation and no notification about it, and no record of what was changed except for the comment about the photos needing to be fixed. IDers do not always immediately mark the observation as containing different species in such cases (because the observation becomes casual and if the observer does not comment that they have edited the photos, nobody is informed that the issue has been addressed and thus the observation remains casual forever).
My second point was that there are some circumstances where people might wish to swap one version of a photo for a better version of the same photo and would likely be unhappy about being prevented from doing so. While it is not ideal, changing photos in such a case does not have major negative implications for the ID (though it might cause confusion if there are comments about photo quality).
Locking photos doesn’t solve these situations because it doesn’t address the underlying issue – namely, that iNat does not record edits to observations and it does not generate notifications for edits even in cases where it might be desirable (such as some of the photos being changed).