I think you are misunderstanding. It will be $15 for the first bag if you check-in online or $20 if you check-in at the curb. People will not want to pay either.
I am not misunderstanding anything.
It counterbalances.
When there is a single $15 charge, the passenger thinks: "There is no need to prepay. If I do need to check-in bags, I will do it at check-in."
When there is a two-tier charge, where it is more to decide later, the passenger thinks: "I'm not sure if I will need to check-in a bag. But I might have to. Maybe I'll play it safe and book one bag in advance."
It therefore creates a counter-balancing effect. This is why the airlines are doing this two-tier level. Rational consumer behavior under the one-tier structure leads to paying the baggage fee at check-in. Rational consumer behavior in the two-tier structure leads the consumer to asks to place a value on the chance that the consumer will not check-in a bag. If that value is $5 or more, then they will pay the fee in advance. But since consumers tend to place a high value on saving money, many will see that value as greater than $5.
Excellent idea, I applaud them. The amount of people trying to sneak on bags that are obviously not carry-on onto planes has gotten out of control. Its funny people complain about baggage fees, but also complain that fares aren't cheap enough when airline fares are cheaper today, not only adjusted for inflation but also in
actual dollars than they were in the 1950s, 1960s, 1970s, 1980s or 1990s.
The next appropriate step is for airlines to reduce the size of allowed carry-ons and/or enforce weight limits on carry-ons, as is done in Europe.