Re: existing model, for recsys, as long as the product already exists you have some baseline available, even if it's not very good. Anything from "alphabetical order" to "random order" to "most popular" (a reasonable starting point for a lot of cases) is a baseline model.
I agree that a randomized control is extremely valuable, but more as a way to collect unbiased data than a way to validate that you're outperforming random: it's pretty difficult to do worse than random in most recommendation problems. A more palatable way to introduce some randomness is by showing a random item in a specific position with some probability, rather than showing totally random items for a given user/session. This has the advantage of not ruining the experience for an unlucky user when they get a page of things totally unrelated to their interests.
I agree that a randomized control is extremely valuable, but more as a way to collect unbiased data than a way to validate that you're outperforming random: it's pretty difficult to do worse than random in most recommendation problems. A more palatable way to introduce some randomness is by showing a random item in a specific position with some probability, rather than showing totally random items for a given user/session. This has the advantage of not ruining the experience for an unlucky user when they get a page of things totally unrelated to their interests.