Comment by dragonwriter

That's not a strong moat (arguably, not a moat at all, since as soon as any competitor has any business, they benefit from it with respect to their existing customers), it doesn't effect anyone who is not already invested in OpenAI's products, and because not every customer is like that with products they are currently using.

Now, having a large existing customer base and thus having an advantage in training data that feeds into an advantage in improving their products and acquiring new (and retaining existing customers) could, arguably, be a moat; that's a network effect, not merely inertia, and network effects can be a foundation of strong (though potentially unstable, if there is nothing else shoring them up) moats.