There's plenty of null hypothesis societies to compare with. You'll find a very clear correlation between the outcomes in similar societies that used differing degrees of preventive measures.
It's hard to find a society that took no measures, as the measures so obviously work and no one wants an uncontrolled epidemic, but there's plenty of societies that had differing degrees of catastrophe up to the point where they realized this or started being serious about it. Czech Republic, Peru, Brazil, Bergamo (the latter just being unaware) +++.
Covid initially had a reproductive number between 3 and 6 in the absence of measures - higher for the latest mutations. A cumulative reduction of R of 80% + 40% would make this an R of ~0.25-0.50, but that's assuming 100% compliance everywhere and always.
Indeed, this is what you see in societies that were serious enough but didn't eradicate the virus altogether -- largely no significant epidemic, but wildfire-like eruptions of disease in local communities that don't strictly follow the measures. E.g. classrooms, public transport, homes, pubs/concerts and so on. Norway, as a case in point, currently has an R of 1.33, with measures that kept R cleanly below zero until the British mutation became dominant (through initial seeding through import and then a few almost-inevitable cases). Cities where measures can almost always be followed have almost no disease, the illness only spreads in areas where many people live close together and have children/teenagers in school. This alone is enough to threaten the capacity of intensive care.
Personally I don't really think this merits much debate anymore, if the objective is to seek the truth rather than some ulterior motive (e.g. politicians who wouldn't mind if the pension liabilities fell). It's not subtle if you actually dig into the details.