I'm pretty sure E3 started being rote 2 decades ago.
In any case it took a big down turn so the industry agrees/agreed with you contrary to the downvotes. Big publishers went off and did their own events at the same time at a different location.
I think E3 is still important, though, because it pulls the industry together. Now whether or not something like GDC could replace it is another thing but I don’t think E3 is any more rote than CES.