The rapid demise of Gotham was stunning — the operatic equivalent of the sudden death of an outwardly healthy person. The troupe had seemed to offer a new model for opera in the 21st century: It gave critically acclaimed performances of small-scale works, often sharing the costs with other presenters, in locales as varied as the Hayden Planetarium, a louche downtown nightclub and the Arms and Armor Court at the Metropolitan Museum of Art.