I don't see E3 going anywhere anytime soon. Nintendo treats it like a marquee event, even with Nintendo Directs every two-three months. Microsoft uses it as a big platform with 1 1/2 - 2 hour conferences every year. I think Sony found themselves in a weird position where they had nothing new to announce (second year in a row), so they decided to avoid to avoid negative press. The other publishers/developers were there for the most part outside of the usual missing suspects (Rockstar)
Outside of Gamescom and The Game Awards (thank you again
Geoff Keighley) , there really isn't another marquee event on the calendar. Yes there is the Tokyo Game Show, but that is a more regional focus. Pax has lost it's luster since there are four of them every year. Even in this day and age where information comes constantly via the internet and you can make an impactful announcement, there is that special charm with E3.
Call me sentimental, but I think it is great that we have a week every year where our favorite hobby is celebrated globally.