I agree, there really are no such things as shoo-ins. That goes for Ridley as well. He may be playable, and he may not be playable; at this point, we cannot definitively say one way or another.
What we can do, however, is analyze what information has been presented to us and form conclusions based on the evidence. In the case of Ridley, we know that he was teased as a boss, yet never confirmed as one, despite Sakurai almost always immediately clarifying what a character's role in the game is as soon as they're shown in any capacity. We also know that previous newcomers, such as Palutena, were teased in a somewhat similar manner prior to their reveal. In addition, analyses of Ridley's shadow, while not 100% accurate, shows that Ridley may be at a size suitable for a playable character. And to top it all off, Ridley's behavior in the Direct does not match up with that of a typical Smash boss; in fact, if that is a boss, it would seem to be a rather poorly designed one.
Putting all these points together, it seems to me that Ridley is likely going to be playable. However, I cannot deny the fact that Ridley was teased as a boss, and for all we know, Sakurai could just be teasing a small, poorly designed boss that people have wanted playable for years. That possibility cannot be ignored. However, I find it to be the less likely possibility.