Bill James is absolutely right, of course, but you aren't supposed to say this. The best evidence that they are replaceable is that they all are replacements for someone else, year after year, and everyone currently playing will someday - someday soon, usually - be replaced. You will notice that no one is answering his point as he tweeted it, they are responding to things he didn't say which are easier to refute. He didn't say that stars are no better than replacement players. He didn't say that baseball was better when they had replacement players (or umpires). He didn't say that fans don't treasure stories about individual players or that they are indistinguishable.
Ask a high school coach if players are replaceable.
But if Babe Ruth, Jackie Robinson, Roberto Clemente and Mike Trout had never existed, we would just tell stories about other players instead. Athletes (and most fans) don't understand what their role is. We pay them to pretend to be heroes (or anti-heroes, sidekicks, villains, archetypes). If the strike zone were different, we would be praising different heroes, but nothing else would change.
I love baseball history and its characters more than 99% of Americans. But I don't kid myself about it.