IMO here is what will happen:
1. The airlines will hide behind this and gradually (or even quickly) move to adopting this standard as it will clearly line their pockets sticking our old fashioned acceptable cabin bags in the hold.
2. The luggage manufacturers must be laughing all over their faces
A completely new market re-tooling frequent flyers with new cabin bags
3. The several hundred pounds I have recently spent on two 'life-time guaranteed' Briggs & Riley cabin bags will be money down the drain. If I need a checked bag they will be too small, and under these guidelines they will be too big.
In short, the airlines are expecting us, the passengers, to pay to solve a problem of their own making.
Many of us, like me, don't carry cabin bags as an 'extra'. Luggage for a business trip of 2 or 3 days can easily be accommodated in a reasonable bin-fitting bag. But take a look at just how slim 7.5 inches is! Even the British Airways additional 'personal item' would not fit this ludicrous new proposed size.
Class action suit anyone?