Storage being cheap is irrelevant..when a user requests the data to be deleted. You delete it. Outside of government compliance there is no reason to not comply with that request
Such an ethic creates a moral reasoning to not comply with an individual's wishes in the immediate deletion of data.
(FWIW I'm not defending this position nor suggesting it's the case here, just you said there's no moral reason that can support it, which seems wrong; different ethical systems can provide different reasoned moral outcomes.)