The initial Wuhan outbreak is completely inadequate for estimating the mortality of an unchecked wave. It was weathered with massive outside help, being surrounded by the most powerful and agile economy in the world and all that. The rest of the planet is hit concurrently, there are no unaffected regions left who could send help. And that local wave was successfully interrupted long before reaching anything close to herd immunity, so peak medical demand was still far below worst case.

Inside the Wuhan quarantine zone the ability to actually test for the disease was extremely overwhelmed. I know there's a paper somewhere estimating the actual number of cases and actual mortality rate based on the infection rate amongst people repatriated from Wuhan, and it wasn't that far above the ~1% everywhere else has seen.

Everywhere else? Germany is the only country at or below 1% for top countries with the outbreak. China only had 4% Spain has 6% Italy 9% the US climbed to 1.3 this morning and will probably climb.

I would consider my number to be an upper bound.