Viruses tend to evolve over time to be more benign - as we've seen with another retrovirus (RNA virus), covid.
A pathogen's ability to cause a disease is NOT related to the symptoms it produces. What tends to happen with airbone infections is that weaker strains (with weaker symptoms) are more likely to spread successfuly because they don't affect people's routine. Stronger strains are more likely to make people too ill to actually go out and infect others. Having weaker symptoms is a key to keep the virus circulating, this does not always mean it is more or less harmful.
Take HIV as an example, it basically has no symptom at all. You can live a whole decade completely unaware before getting ill from what HIV has done to your immune system in the past years. HIV is a model of efficiency and there is no evolutional basis for it to become less dangerous or to develop symptoms.
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Surely HIV has done it a little? surely it's less malign by now, than it was in the 80s and 90s when killing everyone?
This has nothing to do with the virus itself but rather with the drugs used to treat it. There always were reasons to finance new drugs, but prejudice had a big impact on this, both politically and culturally. It took a lot of time for effective drugs to be developed. The first ones did the job but also had enormous side effects so there were lots of people who just prefered to not take them as they'd feel less sick not taking anything...
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If that's true: Why wouldn't people monitor viral & T or CD4 levels, dial up meds whenever they start to go wrong, but dial back (giving the body & the person a break) if they clearly go the right way?
This question I don't fully understand... If you're referring to drugs that are administrated on a six-monthy basis, for instance, there are lots of factors that impose a burden, like it being too expensive or requiring drug levels that could be harmful to our bodies. It being a possibility or not is up to technological advancement and finnancial investment of course. Currently there are trials taking place for PrEP drugs that are administered on a monthly basis or three-monthly basis.
