As an alternative we can auto-save the things to edit automatically in IndexedDB (used by Edit Recovery, but need to generalize to support non-Wikitext edit mode). As a prospect I even supposed that core Edit Recovery should take over the current recovery feature in VisualEditor and DiscussionTools.

The purpose of the warning is actually twofold:

• Avoid users losing their work by accidentally closing the tab, since if the tab is closed, the work is gone.
• Avoid users losing their work by purposefully closing (and never re-opening) a tab on which they think all their work has been saved.

The purpose of auto-save is also twofold:

• Avoid users losing their work by accidentally closing the tab, as the work is restored when they re-open the tab.
• Avoid users losing their work due to power outage/browser freeze/etc., as the work is restored when they re-open the browser.

The first points match, but the second points don’t. So while auto-save could take over some jobs of the warning, not all (and conversely, while the warning helps in some situations where the auto-save would also help, not always). If auto-save is implemented first, it would be a huge improvement compared to the current situation, but the warning would still make sense.