I had a business which was obliterated by WordPress including it's functionality in core (it was menus). My plugin became mostly irrelevant overnight, as the implementation in core was much better than I had in my own plugin.

I saw that as more of a failing on my part than anything else though. If my plugin was good enough, then I would have gotten a lot of kudos and advertising purely from having that functionality bundled into core. I would have been the goto person for menu stuff in core. As it stood, I was that guy who made a half-baked plugin that sort of did what people wanted, but not quite, then core got menus and my plugin and services became irrelevant.

My point being ... if your plugin is good enough, then I think there are benefits to having your stuff rolled into core.

If your plugin is not good enough, and core implements something better, then you just become a sheep who got squashed during the process of WordPress improving itself. Everyone benefits (well, apart from you perhaps, but you are just one person in a sea of millions).