Except we do have data, that’s my point. China’s study is being cited by most experts as this situation unfolds. It doesn’t mean these numbers will hold — it’s possible that what we see out of South Korea over the coming month might be more representative — but it’s what we have, and it’s not nothing:

http://www.cidrap.umn.edu/news-perspective/2020/02/study-720...

The data can't support your 20% hospitalization claim though, and no one in public health is using that study to make that claim. The study itself makes no such claims that this study is in any way applicable to the general population.

About 40% of the patients in that study weren't even tested. They were included based on symptoms that include severe pneumonia. The ones who were tested, sought testing because they displayed symptoms severe enough to seek treatment.

This is the very definition of selection bias, and there is absolutely no reason to pretend that a 20% hospitalization rate is a realistic outcome.

Lots of people on the ship that tested positive showed little to no symptoms. It's very likely that there are a large number of infections that have gone undetected. Until we test everyone we won't know the true number.