An interesting problem facing many people with specialized knowledge is pigeonholing. The problem is pervasive especially in engineering. Why is that? and how does one avoid it?
I get contacted from recruiters from once in a while and that prompts me to look into what different jobs are out there (and for fun, what kind of skills are in demand for high paying jobs). From the engineering perspective, the number of different jobs as an engineer is nearly countless, all of them looking for specialized skills requiring 'X+' number of years of experience.
These job descriptions are misleading, which is the problem-- they lead the reader to believe that if you worked in a field for X number of years, they'll eventually have these kinds of skills. If you're lucky and so happened to work in a manner that led you to have exactly those skills after X years, then you'd have to be really lucky, but for the rest of us we likely won't have that kind of experience.
Keeping yourself relevant in the job market as an employee or as the kind of person that would start a business is a tricky game, and the most important thing is to not believe everything that you read about a job posting. You don't have to be a perfect match, or even an 80% match. You just have to be ahead of other applicants and the better choice than what they could have transferred into that position internally.
I have seen what an internal transfer in a large company looks like; it could be a person from a very different field that they'll be training from 30% to get them started up. Not all engineering jobs are so specialized that they are impossible to train yourself to do. I've seen this in both engineering and financial companies, all that is the most important for you to be is to look like the kind of person that can execute in the position.
Getting through the through the resume filtering process and acing the interview is less about talking about what you have done in the past, but more about the challenges you expect to face in the new position. The more you're able to imagine the challenges that you'd expect to face and the solutions that you can come up with, you've pretty much hacked the pigeonholing problem-- instead of using your skills to do the same thing over again, you're finding new ways of applying them.
Many interesting people with fascinating/successful careers never took the b-line that would eventually lead them to what they are doing as they've changed tack multiple times through out their lives (do a search you will find plenty enough). The challenge is to always be looking forward, look to create value somewhere new and ignore the 'X' number of years in the job description.
