In December 2012, the World Health Organization (WHO) reported 314 samples of the 2009 pandemic H1N1 flu tested worldwide have shown resistance to oseltamivir (Tamiflu).[172] It is not totally unexpected as 99.6% of the seasonal H1N1 flu strains tested have developed resistance to oseltamivir.[173] No circulating flu has yet shown any resistance to zanamivir (Relenza), the other available anti-viral.[174]
The treatment plan at the time included Tamiflu/Relenza/experimental third thing (FDA approved for flu treatment in adults since 2014)
If oseltamivir (Tamiflu) is unavailable or cannot be used, zanamivir (Relenza) is recommended as a substitute.[50][168] Peramivir is an experimental antiviral drug approved for hospitalised patients in cases where the other available methods of treatment are ineffective or unavailable.[169]
I think 2009 H1N1 is a good example of how things could go, as it happened in the modern day.
For a treatment besides Tamiflu: https://en.wikipedia.org/wiki/2009_swine_flu_pandemic cites the who and CDC stating that H1N1 developed resistance to Tamiflu but not Relenza
The treatment plan at the time included Tamiflu/Relenza/experimental third thing (FDA approved for flu treatment in adults since 2014)
I think 2009 H1N1 is a good example of how things could go, as it happened in the modern day.