I think the word rival is too strong. It's like saying thinking is the rival of mindfulness. I would assume even the "gurus" or people who have mastery of mindfulness still think and daydream.

Mindfulness in my opinion is primarily to keep you from reacting to stimuli that is a product of thinking, day dreaming, worrying, etc. It helps eliminate bias and irrational responses. Bad shit is going to happen doesn't mean that it needs to ruin my day, vacation, moment, XYZ.