I count 4 for us: Horford, Smart, Brown and Rozier
I don't think it is sad at all. Some of the change is from good young players coming into the league. Good player movement means teams have opportunities to make themselves better.
I'm not sure what the OP means by "sad," but I do think it's sad in that it's dang near impossible to keep a good team together for a long time. I mean, the Warriors have hung onto their core for, what, 4 or 5 years (and miraculously added Durant)? But that's a rarity. It would be amazing if Boston could pull it off over the next 5-7 years.
Just when guys are getting really good, their rookie deals are running out and teams have to pay them a ton to keep them, and they also have to pay a ton to get good free agents. But if teams do that, they get into the luxury tax and the repeater tax. Things definitely seem geared toward producing a form of parity, in which no team is really good for more than 4 or 5 years, every year sees the rise (if not into title contention, then at least into playoff relevance) of at least a couple of teams (and the fall of a couple others), and it takes having a genius GM like Danny to be able to contend without having one of the league's top 2 or 3 players.
Good thing we have Danny.