The phrase 'soda is a problem but...' likely introduces a problem-and-solution argument rather than a simile or metaphor. It suggests an acknowledgment of an issue with soda, which might be followed by proposed solutions.
The phrase 'soda is a problem but...' does not exemplify a rhetorical device such as a metaphor or a simile, which involves comparisons with the use of 'like' or 'as' for similes, or stating that one thing is another for metaphors. Instead, this phrase suggests the beginning of an argument that may employ a problem-and-solution structure in which the speaker recognizes an issue and then possibly suggests a remedy or an alternative perspective.
Similar to the model paragraph provided on combating hate speech on social media, which outlines solutions for the identified problem, this phrase could be a lead-in to a persuasive argument where the speaker addresses possible fixes or mitigations to the 'problem' of soda.