I think the carry on baggage issue is exacerbated by the airlines themselves. Firstly, exit and bulkhead row pax have no room to stow hand luggage beneath the seat in front, so take up more space in the overheads than otherwise normally taken by pax in other rows. Secondly, of all the available space in overhead bins, some of this is taken up with airline safety equipment and extra pillows/blankets etc. So assuming all pax take two pieces each, there is no way it is all going to fit in the overheads. Even once piece each is going to be a challenge.
Flying MEL-SYD last Friday, I was allocated 35J. On reaching my seat, I discovered that there was no room in the overhead locker at all. The FA kindly indicated for me to stow my luggage above row 32 (empty at that time). On arriving, the reason for all this unfolded.
A couple occupying 35E and 35F (exit row seats) had between them, one soft sided bag that was at least 70 x 40 x 20 cm (I have a good idea because I use a similar bag for my sports equipment). The other had a wheelie which was too big to fit in the overhead above row 35, so was moved back into an overhead locker further behind me (apparently side overheads are bigger than middle ones). The soft sided one was already stowed in another overhead behind me (on landing, the owning pax actually asked the pax below his bag to kindly pass it to him!).
So I have to deduce that all the other pax who had their overhead locker space taken up by these two, were forced to use overhead locker space not necessarily above their seat. Thus general mayhem, not only when boarding, but also on arrival.
Not on this flight, but I have also witnessed a few pax who take on two bags and place both into over heads bins (probably to give themselves extra, precious, admittedly, leg room) - and often not the bins above their seats. I find this really irritating, because it just adds further to the overhead bin shuffle.
So, I guess a pedantic pax would insist on airlines ensuring compliance (and I admit that once, I have had my carry on sent to check in on a DJ flight), and also, that overhead space should be allocated to the pax below it, as a first principle. I don't know what the increased cost to the airlines would be to follow this, but it would surely eliminate some of this farcical behaviour. Maybe there will be a day when there are no overhead bins available at all or a carry on luggage surcharge introduced (although this latter proposal would not necessarily eliminate overhead bin shuffle).