I downvoted for an opinion-based answer. Yea generally viruses evolve to be less deadly and more infectious, that’s the trend (but certainly not a law), but the important thing is the time scale. Plus there are endemic viruses that are not benign. So this comment came across as a bit too confident. Feel free to expand on it.
With regards this claim, the UK Scientific Advisory Group for Emergencies (SAGE) addresses it in their recent paper about future evolutionary scenarios [0].
Scenario Four: SARS-CoV-2 follows an evolutionary trajectory with decreased virulence
Likelihood: Unlikely in the short term, realistic possibility in the long term.
Which makes sense as most transmission occurs in the presymptomatic phase, there's no selection pressure to evolve to be less deadly.
As far as I know the virus H1N1 itself didn't really evolve in the sense that it got milder - all hosts either developed immunity or died off. Now our immune systems encounter one of the endemic, seasonal strains of H1N1 when we are young (or we get a flu vaccine) - however from time to time a new strain of H1N1 emerges that is dissimilar enough, see e.g. 1977 flu or the 2009 flu outbreak.
The original SARS had a much higher mortality rate, which meant that trace-and-isolate responses actually had a chance of working. The mortality rate of Covid-19 is nowhere near that of SARS, which actually allows it to kill more people in the end because it can spread stealthily, and no lockdown or trace-and-isolate has worked. There are also more reports of animal reservoirs for Covid-19 than for SARS.
