The difference is mountain biking or swimming or snowboarding outdoors is fun while using indoor cardio equipment and doing calisthenic type CrossFit training is just work. Indoor circle track riding would be cool though
I'm not going to disagree with you on the outdoors sports- they certainly are fun. But all of weight training is just work too- and can be quite boring. So any variation, like what Soul Crusher does, is bound to be interesting.
Indoor track racing is definitely a fun thing- I've only ridden track outside though on concrete velodromes. In Europe there are many indoor tracks and it gets to be quite a party for both spectators and cyclists:
