But that's missing the point of the rule. In the event of a rapid decompression would you realistically be able to overcome the startle factor and get your emergency mask on within the time of useful consciousness? Probably not.
And that's before the question of whether you will actually be able to breath if the mask has gone into pressure breathing mode. If you have never practiced it then the chances are very low.
So in an aircraft without automatic emergency descent, in the event of a rapid depressurisation above 40k your chances of survival are slim.
However, as has been pointed out, the manufacturers have had to show that the chance of a rapid decompression (that isn't catastrophic for other reasons) is extremely low, so you would think that in aircraft with automatic ED mode then the requirement is unnecessary.