By Dawn Chmielewski (Reuters) – As David Zaslav prepares to take over as head of Hollywood’s new powerhouse, the soon-to-be formed Warner Bros Discovery, he’ll confront a challenge few anticipated last May, when the $43 billion merger of WarnerMedia and Discovery Inc was announced — how to make streaming video as profitable as the old TV business it’s replacing. Nearly two years after AT&T Inc’s WarnerMedia division launched its HBO Max streaming service in May 2020 to challenge market-leader Netflix, Wall Street has grown skeptical of streaming’s long-term prospects, citing the significant a…