While the data are much too sparse and noisy to give an answer about what is happening to COVID-19's virulence (the technical term for the "deadliness" of an infectious disease), or to forecast what will happen to its virulence in the future, there are indeed theoretical reasons that one might expect the virulence to decline in the future.
There is an entire theoretical literature on the evolution of virulence (e.g. see Read 1994, or see Wikipedia); Robert May and Paul Ewald are two researchers who contributed seminal ideas. The basic idea is that infectious 'organisms' (including viruses) may face a tradeoff between their ability to produce lots of infectious particles (e.g. by replicating faster within the host) and the length of time that they can stay in the host before it dies or activates immune defenses sufficiently to end the infection. In this case, some intermediate level of virulence may evolve.
Evolution of virulence in ways predicted by the theory has been observed in the real world in such diseases as myxomatosis (a viral disease of rabbits). The theory has also been suggested to apply in HIV and syphilis.
The actual forecast can depend on many details of the biology and epidemiology of a particular organism. If COVID-19 happened to have a higher-than-optimal virulence in humans at the time of emergence, we could expect its virulence to decline over time. It could also decline if epidemic control was applied in such as way as to differentially affect more-virulent strains; this has been suggested by Tang et al. on the basis of the pattern of mutations in different strains, although their logic has also been criticized on Twitter by an expert in genetic epidemiology, and now by a more official rebuttal by McLean et al. (with some back-and-forth discussion between the authors and critics).
This explanation doesn't include the possibility of host evolution, which is theoretically important but not really relevant to COVID-19 — it would only happen over many generations, and only if the epidemic was so severe that it was a significant overall cause of death or failure to reproduce ...
- Tang et al. "On the origin and continuing evolution of SARS-CoV-2", National Science Review, nwaa036, https://doi.org/10.1093/nsr/nwaa036 .
- Read, Andrew F. "The evolution of virulence." Trends in microbiology 2.3 (1994): 73-76.