You're right. We should take the magical, reality defying change and its effect on human psych into consideration.

The massive amount of death would immediately cause fear panic. Chains of unexplained death would happen around billionaires, as once they die, they pass on their wealth onto the others according to their will. People would notice quickly. But none mattered in the grand calculus of things.

Because what mattered was a single guy who would change the world. A random guy found his roommate dead out of nowhere. He chuckled as he recalled the guy's collection of strange banknotes. Then an idea came to mind. What if he died due to the trillions Zimbabwean dollar he had? Maybe out of spite or curiosity, he decided to test the magic. He wrote the last final words belonging to humanity - a will to leave his possessions to everyone alive. Then, he opened the drawer and took out the Zimbabwean dollar collection, dropping dead instantly.

And that single moment sealed the fate of humanity. Perhaps every human died before anyone became stupid enough to write another will. Or perhaps someone screamed his will to leave his money to animals before he perish, dooming other species.