I can see this being a huge blow to E3, companies will have based their strategies around the shows dates so be forced to drop the news other ways. Presuming the obvious way is to follow Nintendo's lead delivering a digital reveal as they won't want their own press conferences. If it goes well, which considering we are likely getting ps5 & Xbox details it probably will, I'm not sure they'd want to go back to the big show.
It'll be very interesting to see how the companies deal with it, the no hands on time is the hardest thing for them to overcome but if something like xcloud is ready sticking some series X demos out to Xbox one owners over that would be incredible. Technically the one s can output 4k HDR so streaming that to those that could handle it would get a lot more people hands on than E3 would have without the queues, showcase both xcloud and the series X and probably cost them less than the E3 booth and press conference.
Weird thing is I see the need for the show itself for the industry to get together. It creates a buzz for everyone to ride on and allows those in the industry to network and work on things. However E3 recently had steered away from the industry in favour of celebrity's, influencers and advertising to the attendees which has just annoyed everyone and meant their event is now surplus with the industry using GDC more to mingle and showcasing year round visa YouTube. Had they not moved the event in this way it would still be relevant.