I think you're confusing the real-world version of you that has to pay $20 per transaction with a hypothetical billionaire version of you. People moving large amounts of money around don't do 400,000 transactions at $20 a pop.
I'd love it if they simply re-issued the 2013 model. I have a 2013 MacBook Pro and it has been great. Every iteration since then has been worse (keyboards, batteries, touchbar, etc).
Logic Pro is the only thing that might force me to by an inferior current model Macbook if my 2013 model croaks. I'm about to start experimenting with Ardour to see if my Logic Pro dependency can be removed.
They definitely have an incentive to take advantage of Google. If an airline regularly drops the price, and Google keeps refunding the price difference out of their own pockets, word will get out. People are going to preferentially book on that airline, knowing that the day before the flight they're going to get a big rebate.
That brand loyalty will remain even if Google kicks them off of their platform. Moreover, as more airlines get kicked off, Google is left with a smaller and smaller subset of airlines it is brokering tickets for.