As a developer E3 severely messes up your development cycle with 6+ months of development time eaten up by trying to create a game play segment or trailer worthy clips when you don't have a polished final part of game, resources are pulled from your area to get this thing ready for this specific date it's always a hindrance to your project timeline. Not having a trade show that is set the same time every year helps with this and you can show things when they are ready, or a publisher can set and event when they have enough games all ready to show.
Also added to this E3 costs a huge amount of money, and there is a dice roll as to who is seen to have ''won it'' all sorts of reasons it could go badly for a company. Wgy risk this kind of negative reception if the other company pulls out a megaton or a few other trailers?
Would be interesting to see the metrics of people who watch it live vs the people who watch the stream or just even the trailers by themselves else where (due to convenience or time zones etc). If the ratio is strongly in favor of less people watching for the first time live then why go to all that expense and not just have your own stream. I am not suggesting that live shows like E3 are not fun or are dead or that Sony will forsake them, I am just trying to look at it objectively as to why someone like Sony permanently skipping E3 is probably not a bad idea. A mix of state of play style events and one off shows whenever they want seems to be a better bet. Also if it doesn't work out over next few years E3 organizers are not gonna stop them from returning.