As per the 'Customizing Dialogs' section of the link above, when trying to customize core component dialogs, the dialog should be copied over to the proxy component (up to tab node) but should be hidden using Sling resource merger. A duplicate dialog should be created where the actual changes should be made.
What is the rationale behind this? As per documentation, it is to 'To be fully compatible with any changes made to a dialog on its current version' but I'm not fully clear on this.
Each dialog has a consistent node structure. It is recommended that this structure is replicated in an inheriting component so thatSling Resource MergerandHide Conditionscan be used to hide, replace, or reorder sections of the original dialog. The structure to replicate/copy as it is from core components is defined as anything up to the tab item node level.
To be fully compatible with any changes made to a dialog on its current version, it is very important that structures below the tab item level not be touched (hidden, added to, replaced, reordered, etc.).