In my opinion "set" is a better name for what "assert" used to do, and I think "set" was available since it was being used for a low-level operation for which in user code "=" is a better and shorter name.
"I worry about releasing it, because I don't want there to be forces pushing the language to stop changing. Once you release something and people start to build stuff on top of it, you start to feel you shouldn't change things. So we're giving notice in advance that we're going to keep acting as if we were the only users. We'll change stuff without thinking about what it might break, and we won't even keep track of the changes."