If the natural evolution of viruses is to become less threatening, why is the mortality rate of rabies effectively 100% without treatment? Why is Ebola so devastating? Why is MERS still a lethal killer, despite being a coronavirus?

It's because there isn't necessarily evolutionary pressure on a virus to become less lethal. Some viruses have become less lethal over time. But, again, there's nothing that says they have to. Mutations that make them less lethal might also make them less transmissible, for example. For another example, they may only kill you after you've had plenty of time to transmit the virus (like SARS-CoV-2). For yet one more example, they may cheerfully survive as minor diseases in a reservoir species, even if they burn out in humans.