I don't think that's an unrealistic question. I've known plenty of people that have had to leave active duty before getting to 20 years, either because of high year tenure (enlisted) or multiple FOS (officer). Yeah, it's the exception and not the norm, but it does happen.
To the OP: If you're at least an average performer who doesn't get in trouble and doesn't develop a medical issue, there shouldn't be anything other than your own desire to leave that keeps you from making the Navy a career.
I don't disagree with you but as you pointed out perfectly it's the exception and not the norm. The VERY RARE exception. If the person is completely useless they may experience it quickly. But most of the scenarios, in which this occurs, are typically 12-15 years away from the time someone gets in the military before they start getting forced out if it comes to that. By then that individual should realize it and be pushing harder to get to that next level so they no longer fall in that category. I never met anyone that fell into the category of being forced out. That being said I've seen the early retirement.