I passed the PhD, but I was in the same situation as you as an experimental physicist at the beginning of the post-doc phase. During your PhD you have to focus on your specialized niche-topic to graduate successfully and in time, the post-doc phase (especially if you want to stay in academia/get tenure) requests a different mindset and seeking and taking also calculable research risks by broadening the type of research questions you want to tackle and interdisciplinary approaches you want to undertake. Still, your PhD work might be so prospective that you can work the next decade on this topic, but if your feeling says, and you should now have more expertise and overview on this topic than your advisor, that there are no fruits anymore on the tree, I think you have to look for other scientific challenges in and around your field of expertise.
In my post-doc phase I'm also actively looking for questions and applications where I can apply my acquired experience by reading journals and attending conferences I didn't even know of during my PhD. I also visited other research groups at my university to know available instruments and took a look at their recent papers for possible collaborations.
My point is, you have to go deeper AND more interdisciplinary with your scientific approaches and questions. But a successful strategy will depend strongly on what you did and your field. Many physicists became chemistry professors and won the chemistry nobel prize, not because they intentionally switched over to this field or took chemistry courses, it just happend naturally that their experimental expertise paid out very successfully in solving theoretical/eperimental/application questions in the field of chemistry. I see also many professors with a PhD in physics in microelectronics or sensor systems departments. But you will not find many PhD chemists/engineers being physics professors. I just tell you this to prove to you that broadening your scope of interest is in my opinion crucial to stay in academia and be successfully in it. In my opinion your advisor is very wrong here. Physicists have a very broad education, but are often unable to solve any real-world problem in comparison to a engineer after bachelor/master degree, at least in a fast and in the best way of the state of the art. But, especially for interdisciplinary high impact research they are often much better educated and trained.
I don't think you can become professor of mechanical engineering by being solely an expert of 3D printing. Professors are hired for the next 3 decades. Maybe then we have molecular self-assembly ;-) Some scientists become professors with a very specialized but narrow expertise due to current trends in technology, but I'm sure it is by far the minority and getting there is like winning the academic lottery. Therefore, you have to look for prospective fields and questions linked to your expertise under any circumstances.