What mild 2009-2010 outbreak are you talking about? If you’re talking about the H1N1 flu, ask anybody who worked in a hospital at that time. They were slammed, not so badly as peak COVID, but it was pretty rough.
The 2009 flu was a novel version H1N1. The reason it wasn’t worse is that the drift versions from H1N1 Spanish Flu had been circling the globe for nearly a century. Most people have had it once, so the body has some degree of protection.
The reason viruses tend toward less virulence in general is that evolution selects against a virus that kills its host before it transmits. Viruses don’t really want to kill you, they just want to alter your physiology enough that you spray it everywhere you go. A very deadly virus is just one that’s overdone things and will probably burn itself out quickly. The Spanish Flu lasted an uncomfortably long time in human terms, but it’s a really short evolutionary timeframe.