I work for a local insurance broker in the UK and I agree that you’re very unlikely to see cover for Coronavirus/Covid-19 risks (whether purely medical, or for cancellation/curtailment due to the sudden Imposition of border controls, flight shutdowns etc) on travel insurances any time soon. Cover will come just as soon as the companies have a better understanding of the risks and potential costs and are likely to be very expensive at least at first, although the prices will reduce as the insurance companies acquire more data and are better able to calculate the cost Implications.
Travelling to the US from the UK (or from anywhere else where citizens are accustomed to a substantial state involvement in healthcare, which I believe includes most if not all of Europe) without adequate medical insurance would be insane in my view. Forget the current issues, just think of how much it would cost if you were to trip over a kerb and suffer a compound fracture of an ankle, or if someone in your party was to have even a mild heart attack...
We have our next trip booked for December 2021, secured by a small deposit to our TA back in early January when we all thought the reports from China of a new flu bug were nothing to worry about, with the balance due in August 2021 so that’s our ‘cut off’ point (we could afford to write off the deposit if we have to). DW is really looking forward to the trip and I haven’t had the heart to tell her yet that whether we can make the trip or not may in the end come down solely to the availability/unavaliability of insurance against coronavirus-related cancellation and medical costs...