You can actually board a domestic flight without any ID at all, for example if you lost it before your trip. But you'll have to go through a manual identity verification process. That includes giving fingerprints and answering personal questions only you should know, like past addresses.
It takes around 30 minutes and if you don't answer correctly, you could be denied boarding. This process already existed before the Real ID requirement, but it used to be free. Now, you're forced to go through the same manual verification steps and pay $45 on top of it.
You're being treated the same as if you have no ID with you at all.