Cool, cool. To preface this comment, I'm a user experience designer, and I've been doing web software development and design for over 20 years.
In reality, your conjecture turns out to be largely untrue. Visual design quality does have an impact.
Main principle you want to look up: the Aesthetic Usability Effect. Basically, users perceive your product to be more usable if it's aesthetically pleasing.
> Summary: Users are more tolerant of minor usability issues when they find an interface visually appealing. This aesthetic-usability effect can mask UI problems and can prevent issue discovery during usability testing. Identify instances of the aesthetic-usability effect in your user research by watching what your users do, as well as listening to what they say.
From https://www.nngroup.com/articles/aesthetic-usability-effect/
See also: https://www.nngroup.com/articles/perceived-value/ (other ways users perceive value based on the UI)
Of course, this has limits. A rebrand or redesign that, all other things equal, is mostly the same quality as before probably won't get much benefit. I've seen many redesigns and that tends not to be something people do—there's usually a good bump in visual quality in the process.
So, regardless of whether you, personally notice the different functionality, many redesigns are actually a success at improving the perceived value, perceived usability, and overall evaluation of the brand, company, and product.
People aren't idiots. They don't do this work for no reason. Try to understand and respect the work outside your own department and specialty. Building respect across those boundaries is a rare and valuable thing.