Not a new model. Priceline did it for years and ultimately quit.

Personally I would never feel comfortable saying I’ll pay $X for a flight and not knowing if I’ll end up on Delta or Spirit. I also dislike guessing at an acceptable bid, and the multiple rounds of offers if my bid is too low. Show me actual flights and actual prices and I’ll pick one, but I’m not going to play a guessing/gambling game to save a few bucks. But I suspect I, like many others on here, are not your target market - infrequent, highly price sensitive travelers who just need to get somewhere and have no airline preferences.