Bogle has gone on record that he believes the market efficiency limits of passive indexing are somewhere in the 70-90% range of equities held inside such broad market indexes [1]. A more reasonable and less click-baity headline would have been "Bogle Sounds Governance Warning on Index Funds".

[1] https://www.ft.com/content/4594f554-ba1a-11e7-9bfb-4a9c83ffa...