Honestly I think the coronavirus is probably more of a scape goat for the cancellation than the big reason for it. They'll claim otherwise but considering many other events are going on right now without issue despite the outbreak I doubt they'd make that decision now if that was the only issue. In reality with the high profile drop outs recently E3 seems to be fucked behind the scenes right now. I'm guessing the reception of the E3 2020 plans and the doxxing has caused a huge management shake up which is probably screwed their chances of hitting the deadline, so they're using the virus to save face, at least that's what the conspiracy side of me's thinking.
With that said I agree with everyone else that this isn't that big of a blow honestly, all the companies will do their own Direct style thing that will be just E3 but shorter and with less cringe. E3 will probably comeback next year but with only a handful of companies due to how much more cost effective it is to dump them, maybe limp on for another year if they're lucky, than it will shut down for good most likely.