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).