Why it has never, and can never, enter and be succesful against MMA?
Why should it? Last time I checked, real self defense situations do not involve fighting other trained fighters in a ring, with dozens of rules, etc., but unplanned and unavoidable attacks in real life against people not as trained.
Of course, aikido has been highly useful to police forces, including in Japan. You know, the real world, not sport or training.
You overlooked that.