If Discovery fails, I think it will have more to do with it being on CBS All Access than to The Orville. The show could fail to pull in enough new subscribers because of fans unwilling to pay for another streaming service, especially if they have some reservations about the show's new style. It might have been better to put the show on a regular network where they can get more viewers and with good ratings, be a commercial success.
It would have been hard to compete with CBS' other shows in terms of ratings. It would have to compete with other networks, and the ratings of other shows on CBS, to survive. Advertising revenue is based off of ratings for viewers ages 18-49.
Even if CAA only adds 2 million subscribers to its already existing subscriber base, each of those subscribers matter since they directly contribute. And even if they subscribe only the four months that Discovery is on, that is $48-80 million that is direct to CBS. If they don't cancel, that's $144-240 million a year. It'll obviously be somewhere in between.