Who came up 40x30x15? I have a "standard" sized laptop with a 15.6" screen, and that is 38cm x 28cm before you put into a padded "slider", never mind a "proper" laptop or backpack type bag with padding?
And if you are in Y on a A380 in a seat with the IFE box, there is barely enough space for your feet never mind a bag as well!
No way that is going into the hold - so what happens if I refuse to let them do that? Do I get offloaded to a later flight, or just thrown out of the airport?
I guess if they do enforce these rules (that so many people ignore today) then there is little likelihood of them needing to put them in the hold... so it may all be a moot point.