If there's nothing to show, the theaters won't be open.
Also, the parks can't just reopen without operating at a loss. They need to announce a date and have people planning vacations. If you just open, then you're simply counting on some hardcore APs and CMs not working to fill your parks. If nobody has vacations booked, the parks are losing even more money being open than if they stayed closed until rooms started to book up a few weeks out.
I think Disney's June 1 date for hotel stays is probably pretty close to when we can expect the parks to be open at the soonest.