Exactly. The only reason that the other four endemic human coronaviruses don't kill many people today is that most of us get infected as children and build up natural immunity which protects us later. But they can still be deadly to people with weak immune systems.
Obviously most people weren't infected with SARS-CoV-2 as children so the smart move is to get vaccinated and jump start your immune system. Non-pharmaceutical interventions like masks and social distancing are only marginally effective so everyone will eventually get exposed to the virus no matter what we do.
The other influence is the evolution of the virus itself. A longer lasting less-than-lethal infection is, in the long term, is selected for by evolutionary pressure.
In the short term though the biggest selection pressure on the virus is vaccine evasion.
Longer lasting, sure. Less-than-lethal (or otherwise less harmful), not necessarily. Whether the host dies a few weeks after contracting the virus is not really relevant for transmissibility.
Actually, several viruses have evolved to become deadlier:
Some of their numbers are a bit off due to lack of data early in the pandemic. For example they stated COVID-19 mortality rate as 1.3%, whereas we now know that in the US it was only 0.6%. But qualitatively their conclusions still seem to have aged well.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7102597/
Obviously most people weren't infected with SARS-CoV-2 as children so the smart move is to get vaccinated and jump start your immune system. Non-pharmaceutical interventions like masks and social distancing are only marginally effective so everyone will eventually get exposed to the virus no matter what we do.