Currently, merging in Wikidata happens through a combination of a Gadget and a bot. The gadget has to be enabled via the settings. After an item gets merged it takes some time and then KrBot replaces links to the obsolate item with links to the item in which both items get merged.
This hacky way has two problems:
- New Wikibase editions that don't have KrBot, won't get the same merging.
- Undoing merging takes a lot more than one click.
- Given that undoing merging takes more than one click, we make merging harder for newbies by not activating the merging function by default and as a result we get less merging than is desireable. Our new item creation dialog suggests that a newbe who accidently created a dublicate item isn't supposed to merge it but is to request deletion of the item.
Given that WMDE recently made a strategic decision to push secondary Mediawiki installations and a project like FactGrid likely need merging from time to, it seems to me that this task deserves more development priority then it would otherwise have.
Having a functionality that bundles multiple content changes together to one merge that can be undone with one click might need new core functionality. That core functionality might be designed in a way that also always the undoing of a batch of QuickStatements with one click. Even when working on that level is hard because it goes into deeply into what the unit of an edit happens to be in MediaWiki I think it's worth to look at this because it will make scaling in general easier.