Imagine if you will, that the black death had totally sterilized Europe around 1450, no survivors at all. What would have happened to the native Americans with their paleolithic technological base??
My version of an answer to that: It might have taken 100 - 150 more years, but sooner or later, some sort of a combined fleet of Ming/Mongol ships similar to the ships of Zheng Ho would have gotten to California with horses and squadrons of cavalry using Mongol tactics and those powerful composite bows, and capable of crossing North America West to East in about a month and inside of the first six months, they wouldhave had better maps of the Americas than any native group had.....
Or what if the Moors still ruled over Spain. And the Brits were too busy fighting the French?
Maybe the Russians would have arrived. They had settlements in Alaska at the time.
Mongols vs. Comanches ... now that would be interesting.
“… the ships of Zheng Ho would have gotten to California…”
Zheng would probably just passed California by. His fleet was based on the idea of trade and forcing smaller recalcitrant kingdoms to recognize China as their overlords if they refused to provide tribute to the emperor. Since the natives of California had literally nothing of interest to the Chinese, the Chinese would have continued south, likely becoming the Conquistadors of the Aztecs and Inca. Those civilizations at least had trade goods and gold.
The race would have been "on" between the Chinese and the Muslims for the conquest of the New World. My money would have been on the Muslims; unlike the Chinese, they had an imperative to go out and convert the heathen to Islam. Also, they were pretty good seafarers.
The Chinese, OTOH, thought -- with some justification -- that everyone else was some flavor of barbarian, and every other culture was completely inferior to theirs. There was some interest in trade, but only very limited interest in any kind of conquest or colonization.
(Note that NW Australia is far closer to China than to anywhere in Europe or North America, but the Chinese probably never got there and certainly didn't colonize.)