The phrase "much in demand" can be substituted with several synonyms depending on the context and tone of the sentence. One synonym is "highly sought-after," which implies that the item or service is in great demand and is highly valued by many people. Another synonym is "popular," which suggests that the item or service is in demand due to its widespread appeal. "In vogue" is another alternative, particularly when referring to a trend or fashion item that is currently in demand. "Trending" and "in-demand" also convey the idea of popularity with a focus on its current relevance. Ultimately, the chosen synonym depends on the level of emphasis that the writer wants to convey regarding the demand.