The point of ‘Basic Economy’ fares — which don’t let you assign seats in advance, upgrade, or make changes for a fee, and at United and American don’t allow you to bring a full-sized carry on onto the plane — is to make the travel experience so unpleasant you don’t want the fare, and will spend more to avoid the restrictions.
The risk is that customers don’t spend more with the airline offering the inferior Basic Economy product, but instead book a different airline instead (or even just decide not to travel). That’s what happened when United launched their basic economy product. They lost business. And they lost about a hundred million dollars.