If the Speak Mandarin campaign has succeeded after 47 years, why fear dialect movies? Gen Z's and younger no longer speak any dialect. Dialect movies are for the elderly and boomer aunties and uncles who don't have any impact on the Mandarin-speaking scene.
If the Speak Mandarin campaign has failed after 47 years, then #1 time for a revamp or scrap it altogether, #2 a Teochew movie is not going to get all the young people speaking Teochew again.
Either way, no reason to ban dialect shows. No-brainer really.
Taiwan's strong Minnan (Hokkien) tradition has not diluted their proficiency in Mandarin or their command of Chinese. In fact, all Taiwanese high school students learn wen yan wen (Classical Chinese), which is only taught at university level on the Chinese mainland.