The reality is that most economy passengers neither know nor care which equipment they're on when they book their flights.
This depends. Plenty of people are no doubt utterly clueless or indifferent.
But there are plenty of people who are flying more regularly on longer sectors and they do start to care about carriers, airports, and equipment. And they talk to family, friends, and colleagues who maybe don't fly so frequently. That probably adds up to more than you'd think.
BTW - which would you rather be on? A packed A380 or a 60% full 777?
Well the suggestion seems to be that the 60% is more likely with an A380!
The reality is that when people book they have no idea how full the flight is. They might infer from a low pricer that it is not. It's sometimes the sectors you think which will be least full on which you can't select a decent seat. And that situation can be fluid even after you checkin on a two sector flight the situation on the second sector may change due to circumnstances. So this 60% or 100% preference scenario isn't really a choice is it?
And it also depends on aircraft config. If booked on a A380 with upstairs economy cabin with 2+4+2 seating you get a better chance of getting a pair of seats - which may be near the front. Whereas on a 777 typically there are a few rows at the rear with 2 seats at the window.
And then there is issue of whose A380 or 777. Some 777s are 3+3+3 in the back (eg. BA) and others are 3+4+3 (eg. Emirates) and ISTR that MAS had 2+5+2. The lower cabin of the A380 will likely be 3+4+3 but wider than the 777 with the same config.
Personally? I assume my flight will be 100% packed. Travelling economy with spouse if the A380 has a 2+4+2 cabin option then that's my prefered choice. More chance of getting a pair of seats at the window up front up top. But it's not always obvious as to what the equipment is before booking.