My first E3 was in 98' (in Atlanta I believe....or Atlanta was the year after) and I have been to a C R A P ton of them and the steady decline isn't a surprise from the way I see things. IMO E3 was always the "CES" of video games, it was a great place for creators to show off upcoming release and get folks excited. At first you could get in and get a booth with a modest investment, and in the early days the booth's came with power and basic items (a table, some chairs, etc).
Then as the games industry grew in value so did the cost of going to E3. Before the floor fell out in 2014 (which I think was the first year the entire show was open to general attendees not just the last day....cannot remember exactly) it was prohibitively expensive to get a booth space (did not include ANYTHING....not even drapery for areas around you), fly your team out, get hotel rooms, and then spend even more money on getting access to advertise (press flesh with the media) while you are there.
The internet (twitter, snapchat, vine, instagram, youtube, facebook) makes it SO easy to reach out to your community now that for mid to small sized publishers or developers its crazy to spend that kind of dough to show off a game that will in most cases get lost in a sea of other games. Unless MS, NIN, Sony, or a big name publisher is attaching your product to their plans it becomes really hard to get noticed without spending a years worth of Payroll in the process.
As far as the public getting access to E3.....well there is PAX for that. It's much cheaper and you are going there specifically TO MEET your community. At E3 you want access to the media to help market your product and they lost that as a USP for their product offering....you can do it better and cheaper somewhere else.
Don't even get me started on their priority points system for return exhibitors.....whew that was/is a mess.