I’m not sure deadliness is orthogonal to reproducibility. You’re correct that the statement you provide is false, but I think I would defend a similar statement as follows:
1. Causing humans to get sick is very likely to make a virus less transmissible, as the host stops moving around as much, or dies. This generally happens in the short term, but if not then in the long term—for example, if a virus transmits solely through relatives touching the corpses of the dead, it may initially be more transmissible the more lethal it is, but once the human populations that maintain this custom have been replaced by those that don’t, then killing its victims rapidly will become a disadvantage.
2. The disadvantage for a virus in causing humans to have symptomatic illnesses in is in tension with the fact that to succeed, viruses need to make human cells stop doing what they’re supposed to do, and start reproducing the virus, which is by definition going to mean our bodies working less well.
3. All viruses face both of these evolutionary pressures. Together they mean that the deadliness of a viral disease in a human population isn’t random but, for a particular virus in a particular population, has an optimal level.
4. When we notice a virus starting to be transmitted between humans, and becoming endemic in the human population, we do so because that virus is more dangerous than all or nearly all other viruses currently in circulation. By virtue of the fact that we have noticed a virus, it is likely that on the “cause less/more sickness” axis it is further towards the “more sickness” end than is optimal.
5. So the new viruses that we are aware of tend to evolve to become less dangerous.
The most obvious weasel words in the above are “for a particular virus in a particular population”. Given that humans evolve, and human customs and immune systems change, in response to viruses, then it could well be the case that in general the effect of viral evolution is dwarfed by the effect of humans evolving, and human societies and immune systems changing, in reaction to the presence of the virus in humans. So viral evolution might not matter much. Even if it does, other evolutionary pressures on the virus, such as avoiding the human immune response, might be far greater than the pressure to become less dangerous to humans. But I would still expect to see new viruses that we are aware of tending to evolve to become less dangerous.
One angle to look at invention from is the curious fact that so many things are invented by different people in different countries; and that if you look into it you generally find that most of these multiple inventors have a point (rather than, as in Star Trek, Russians just being adorable idiots).
Just from your list, and from a British perspective/quick wiki’ing, Swan invented an electric light bulb that worked well enough to make him a lot of money before Edison—and Turing built the first computer, as opposed to calculator. And I’m sure there are French/German/etc equivalents that are just as accurate, and just as partial. Though even though I rationally know this, and have no conscious desire to defend my nation’s scientific honour, here I am writing this comment.
So invention as an idea (and as it’s normally thought of) is suspiciously connected to tribalism and identity. It may not be much use for describing or investigating how discovery works.