Ask HN: FreeBSD, a realistic choice as OS X replacement? If not, which Linux?

So I've seen the posts about alternative laptops to the MBP (for example https://news.ycombinator.com/item?id=12846516 ). I am now looking at OS alternatives. Given that OS X is based on FreeBSD, wouldn't it make sense to use FreeBSD as a replacement? Or is it just a dumb idea?