As a menace that strikes mostly at homosexuals and intravenous drug addicts, the mystifying disease called AIDS has seemed ominous enough. But its future as sketched last week by the Public Health Service is downright scary. A PHS conference of experts on the disease speculated that AIDS will multiply more than ten times by the end of 1991, the caseload rising from 21,517 known cases to 270,000, the death toll from the 11,713 so far to 179,000. Increasingly, AIDS will afflict heterosexuals, and it will spread fast outside of such hard- hit cities as San Francisco and New York. An estimated 1 million to 1.5 million Americans have been exposed to the AIDS virus, and up to 30% of these could develop the syndrome.