If domestic airlines start changing their carry-on rules to smaller and smaller bags, they are going to have to bear the wrath of the business FF. They are the domestic cash machine for airlines. If they are suddenly told that they have to go out and buy smaller bags, or pay to check their bags--after buying domestic tickets of over $1000, you will see revolts in the making. (Most of them don't buy $30 bags. They tend to spend $200+ of their own money.)
Airlines are cutting back every way they can. Charging for checked bags, charging for food, no more movies on domestic flights, and so on. They are also putting their people near the security checkpoints and at the gates to make sure people don't go over the current carry-on guidelines.
What they would probably do is not cut back on size, but on weight. Most domestic airlines allow carry-on of 40 pounds. That's more than double most European airlines. Weight is what causes the plane to burn more fuel, not size. So, carry-on weight limits would make more sense than size limits.
I predict that the true next step for airlines is continuing the lessening of number of flights. With less supply, the demand will go up as will ticket prices. There will be more restrictions on days of travel. (United just started the Saturday night stay rule once again. I think others will follow.) You will also see the FF mileage requirements go up.
Flying used to be the more expensive choice for travelers. Then, with deregulation came discount airlines and planes became Greyhound buses with wings. I believe we will now go back to the days of more expensive flights with serious restrictions on the cheap tickets. Look for consolidation and partnerships of more and more airlines. And don't expect things to get better anytime soon.