Just a reminder that E3 served multiple purposes:
1) In the days when games were printed ROMs that took months to produce, publishers and retailers would agree in June to the number of cartridges to be made. With discs / digital, that lead time is far reduced / irrelevant, and they don't need to set orders 5 months in advance.
2) Institutional stockholders would also be able to see the upcoming games and adjust their portfolios accordingly. The smaller number of publishers having their own dog and pony shows with stockholders and the much later gold dates for discs / digital have rendered the E3 timeframe moot.
3) Networking - while there are concerns about the blue hairs taking over GDC, etc., there are more modern networking options than just E3 nowadays.
4) PR - while gamers focused on just this aspect and gained the benefit of seeing everyone's lineup simultaneously and could plan on their purchases ("Do I beg for a PS2 or a Gamecube this Christmas?"), no surprise publishers don't want their message getting buried under a dozen other companies' press releases.
So all in all, not a surprise E3 is dead, as much as gamers were able to get the benefit from all the information at one time.