Earlier quoted context omitted.
It depends on what you call severity. If severity means the host dies quickly rather than slowly, there's obviously selection pressure and that mutation will die off relatively quickly vs the rest, since it won't have as many chances to spread as the less severe strain. E.g. SARS/MERS. If severity means the host dies more often, but it still takes a long time to die, the selection pressure is much lower, and it will…
Spanish Flu did evolve to become more mild. All modern seasonal flu variants are descended from it.
That doesn't make sense. We have seen regular waves of flu, some being more pathogenic than others. There has not been a "direction" of flu strains becoming milder. So-called Spanish flu came after weaker strains.
> All modern seasonal flu variants are descended from it.
I don't think this makes sense either. The more serious strains of flu we've had in recent decades have for example included contributions from bird and pig viruses.