Why is ‘Undo’ more important than ‘Delete’?
Grace time between action and finalization
'Undo' is less a feature for reversing the past than a 'temporal slack' that keeps the possibility of modification open until the user's decision is finally confirmed.
Delete as a recovery period
'Delete' also often starts a kind of countdown (for example, Google's Photos 60-day retention), not immediate disappearance, so both Undo and Delete connect to the question of 'when to finalize an action.'
Confirmation dialog vs. undo
Confirmation dialogs that ask every time, "Are you sure you want to delete/send this?" increase user fatigue (friction). Rather than preventing things in advance, letting the user act first and then giving a short grace period provides a more flexible UX.
Different demands depending on the situation
Depending on the user's intent or the sensitivity of the data (for example, personal information, medical records, etc.), immediate and complete deletion may be necessary, so unconditional recoverability is not always the best choice.
Core conclusion
The key is where, and how clearly, to place the point of no return in the UX.
