"the overall market eventually recovers from disasters." is not really true. The American market has always recovered from disasters but globally it surely is not true. In fact, of the ten largest stock market in the world in 1900, four of them went to 0 meaning that you lost all your money and had to start over.
For your original question, you don't mean can the ETF go bankrupt. You mean could it eat all your money. The answer of course is yes. From the ProShares prospectus -
"Further, because the Fund includes a multiplier of twice
(2x) the Index, a single day movement in the Index approaching
50% at any point in the day could result in the total loss of a
shareholder's investment if that movement is contrary to the
investment objective of the Fund, even if the Index subsequently
moves in an opposite direction, eliminating all or a
portion of themovement. This would be the case with any such
single day movements in the Index, even if the Index maintains
a level greater than zero at all times."
I actually think there might be something that would save you in the spot that the prospectus is talking about. Before you lost all your money, you would get stopped out of swaps and get margin calls the fund couldn't meet with futures contracts (which would presumably stop them out too). That means in a 2x fund, if the market went down 50% at least a little slowly you might save some of your money. Maybe...