: Would the human seasonal coronaviruses be just as deadly as COVID-19 in a population with no prior immunity? Presuming that we were able to find a person who was never exposed to any of the human seasonal coronaviruses (part

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The seasonal coronaviruses attach exclusively to cells with a ciliated epithelium.

Coronaviruses invade the respiratory tract via the nose. After an incubation period of about 3 days, they cause the symptoms of a common cold, including nasal obstruction, sneezing, runny nose, and occasionally cough (Figs. 60-1 and 60-2). The disease resolves in a few days, during which virus is shed in nasal secretions. There is some evidence that the respiratory coronaviruses can cause disease of the lower airways but it is unlikely that this is due to direct invasion. Other manifestations of disease such as multiple sclerosis have been attributed to these viruses but the evidence is not clear-cut.
Studies in both organ cultures and human volunteers show that coronaviruses are extremely fastidious and grow only in differentiated respiratory epithelial cells. Infected cells become vacuolated, show damaged cilia, and may form syncytia. Cell damage triggers the production of inflammatory mediators, which increase nasal secretion and cause local inflammation and swelling. These responses in turn stimulate sneezing, obstruct the airway, and raise the temperature of the mucosa.

So, the mechanism of infection is such that they only attack the upper airways.
The SARS-CoV-2 is totally different. It attaches to the ACE2 surface receptor which is found deep in alveolar pneumocytes. Their damage causes loss of surfactant in the alveoli, the collapse of these air cells that perform oxygen exchange and subsequently ARDS.
So, seasonal coronaviruses give you a runny nose but don't kill. SARS-CoV-2 collapses the air sacs that allow oxygenation to occur. www.ncbi.nlm.nih.gov/books/NBK7782/

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I should add here that there has been a case of ARDS reported for the alphacoronavirus 229E (i.e. a "common cold" coronavirus) in 2018. But ARDS occurrences are of course, much less common in these "common colds" than in Covid-19 infections. The case report for 229E discusses the few other such occurrences:

HCoV-229E has been associated with bronchitis, acute exacerbations of COPD, and pneumonia in infants, children, and elderly persons with underlying illnesses [11–13]. Life-threatening infections have only been described in immunocompromised patients [7, 8], but the correlation of HCoV-229E with LRTI in healthy adult individuals is uncertain [9]. An adult patient with pneumonia tested positive for HCoV-229E has been described in a study conducted in rural Thailand, but it is not made clear if other comorbidities were present [14]. Nine Italian patients hospitalized with LRTI have also been tested positive for HCoV-229E; however, their age is not specified [15]. ?lthough numerous studies have tentatively linked 229E infections to severe respiratory tract illness over many years, no study controlling for age and underlying illness has demonstrated an epidemiologic association between infection with HcoV-229E in healthy adults and any illness other than the common cold. Furthermore, no case of HCoV-229E-associated ARDS has been reported in immunocompetent adults. Only a few cases of pulmonary infection and ARDS have been described in a 76-year-old woman infected with the closely related alpha coronavirus HCoV-NL63 [16] and in a 39-year-old woman with poorly controlled DM and infected with the beta coronavirus HCoV-OC43.

For severely immunocompromised patients however, there have been cases of 229E infection, triggering pneumonia e.g. as reported in (the papers referenced from the prviousone):

"Coronavirus 229E-Related Pneumonia in Immunocompromised Patients"

Here we report 2 well-documented cases of pneumonia related to coronavirus 229E, each with a different clinical presentation.

or "Coronavirus Pneumonia Following Autologous Bone Marrow Transplantation for Breast Cancer"

Rarely has coronavirus been linked, either by serology or nasal wash, to pneumonia. We report a case of a young woman who, following treatment for stage IIIA breast cancer using a high-dose chemotherapy regimen followed by autologous bone marrow and stem cell transplantation, developed respiratory failure and was found to have coronavirus pneumonia as diagnosed by electron microscopy from BAL fluid.

But this is like a handful of cases in toto (for 229E).

The ARDS case cited/mentioned in that (first) paper for NL63 was reported in 2016. This was also diagnosed by BAL + electron microscopy. This latter paper also suggests that in immunocompromised patients NL63 may be a bit more problematic (than 229E):

In 2010, a case of fatal HCoV-NL63 pulmonary infection during the late-engraftment phase was reported (6). An Australian study reported that 2% of patients presenting to the hospital because of respiratory symptoms tested positive for HCoV-NL63, and 81% of these patients were diagnosed with lower respiratory tract disease; all of them required admission to the hospital, 56% had an abnormal chest xray, and one immunocompromised patient died, yet none of these patients had ARDS (7).