I think in that situation your design can be effectively stolen - if you can no longer use it, because someone else in your field is using it and your own use would look like plagiarism. They have it and, crucially, you can no longer use it with commercial effect.
You can "effectively" steal a design without the copyright infringing copy/paste. Example: the last-generation Kia Optima that looked just like the Mercedes E series.
I think your example only works if Mercedes created the design and Kia came out with it first. Kia probably wouldn't be bothered with Mercedes copying them.
To follow the car analogy, an exact copy would include the 3-point star logo, which would take you into the realm of counterfeiting which is illegal in an entirely different way.
I think in that situation your design can be effectively stolen - if you can no longer use it, because someone else in your field is using it and your own use would look like plagiarism. They have it and, crucially, you can no longer use it with commercial effect.