> There are evolutionary pressures on viruses that tend to weaken them over time.
Those evolutionary pressures relate to transmissibility. Basically, a virus that kills its host before it can spread doesn't spread very far at all.
SARS-CoV-2's defining feature of asymptomatic spread decouples this and so far hasn't led to weakening – indeed, Delta is both substantially more transmissible and more likely to cause severe disease and death.
Those evolutionary pressures relate to transmissibility. Basically, a virus that kills its host before it can spread doesn't spread very far at all.
SARS-CoV-2's defining feature of asymptomatic spread decouples this and so far hasn't led to weakening – indeed, Delta is both substantially more transmissible and more likely to cause severe disease and death.