I'm booked for another trip to Greece early October and as of now everything is still on including flight, accommodations and car rental. I'm staying in touch with accommodations and car rental people and they have been great and said there will be no problem if I cancel but I'm not doing so unless there is something definite the Greek government does to cancel or extends quarantine into the fall.
As far as hospitals on Greek Islands go most of them have only a clinic with some larger islands having a small hospital. Many of them will not to be able to handle any major virus outbreak and that is why the Greek government has canceled all boats/flights to islands except for locals.
We all have to sit tight for future trips anywhere and hope people follow orders and the virus plays it self out.
I'm staying optimistic and hopeful about my trip and hope the Greek people get over this just like everyone else!