This is an effect with all antivirals, and was predicted from the start. It's much like antibiotic resistance in bacteria. Except it arises even faster as the

This is an effect with all antivirals, and was predicted from the start. It's much like antibiotic resistance in bacteria. Except it arises even faster as the replication and mutation rates are much higher with viruses. In people with HIV, treatment with a single antiviral drug generally leads to strong antiviral resistance within a few years at most. This is why it is always treated with two, three or even more parallel drugs that work by different mechanisms. Developing multiple mutations simultaneously is improbable. Genetic testing of a person's HIV strain before treatment to determine resistance profile is standard now in rich countries. There is one small grace. The traits that improve resistance to antivirals usually decrease overall fitness. There have been cases of patients who have, after a long time, had their strains of HIV lose its resistance to certain antivirals. When treated for a long time by different agents, the mutation for resistance no longer serves a purpose and is rapidly selected against. Anyhow, given the nature of COVID I would look to influenza antivirals for what to expect; they're mostly useless because of widespread resistance.