Your model is missing one big variable and that is latency, the time difference between when people are most contagious and when they are symptomatic.
SARS-1 had low latency, and so people tended to become contagious and symptomatic at around the same time, reducing transmission. Whereas SARS-2 has a high latency which helps it to spread while people are unaware they even have it. This also insulates SARS-2 from selective pressure to reduce its virulence.
SARS-1 had low latency, and so people tended to become contagious and symptomatic at around the same time, reducing transmission. Whereas SARS-2 has a high latency which helps it to spread while people are unaware they even have it. This also insulates SARS-2 from selective pressure to reduce its virulence.