There are seven corona viruses.
Four have been circulating for years causing cold like symptoms: 229E, NL63, OC43, and HKU1. Then there are the deadly ones, SARS, MERS, and finally Covid-19.
SARS-CoV-2 has an error correction system which retarded the rapid viral evolution which normally makes a disease more contagious but also much safer. Crucial interfering mutations in the nsp14 protein described in the spring 2021 (please correct me) have accelerated mutation rates enough to make the doctors’ observations perfectly believable.
Interesting comment. Do you have a reference?
Is it known why MERS and SARS are more deadly than SARS-CoV-2?