How do you persuade your teammates on design change without the metrics?

ROI on UX design has been one of the huge topics for the designers and I often find it hard to find the right answer. Design (especially visual & branding) could be such a subjective topic that everyone has an opinion about it. As a startup designer, I often run into a situation where I have to persuade the teammates to prioritize on design tasks without the hard ROI data. Few things I have done in the past that worked well. 1. Build super quick static prototype and interview couple of users. 2. # of people or # of times that the design is / will be viewed 3. Use well established design principles (https://lawsofux.com) when communicate an idea Have you run into a situation where your design project gets deprioritized due to lack of ROI? I'd love to hear and learn what other designers are doing in this type of situation.