> The fact that explorers 500+ years ago were brutal conquerors
You left out a few caveats. You mean that state sponsored explorers with the mission of bringing in money to the state, including by taking gold and slaves, were brutal conquerors.
Other explorers were not brutal conquerors. You need money to conqueror.
Pytheas, the Greek explorer who first visited what is now Great Britain and Ireland, was not a brutal conqueror.
Marco Polo, 800 years ago, was an explorer who not a brutal conqueror. The same for Ahmad ibn Fadlan, the Muslim diplomat and explorer who went to the Volga Bulghars, and for Ahmad ibn Rustah Isfahani, another Muslim explorer of Europe and Asia.
The Polynesian explorers of the Pacific were not brutal conquerors - there was no one to conquer. The same for the explorers that reached Iceland, the Canary Islands, and other uninhabited places. And for the polar explorers, and space explorers.
The fishermen who explored the Grand Banks in the early 1500s did so for fish, not to conqueror land or people.
The Roman explorers who went to south and east Asia, did not do so to conquer. They did so to trade. https://en.wikipedia.org/wiki/Romans_in_sub-Saharan_Africa points out the Roman explorations across the Sahara "had mainly a commercial purpose. Only the one conducted by emperor Nero seemed to be a preparative for the conquest of Ethiopia or Nubia."
Giovanni da Pian del Carpine, the Italian explorer who entered the court of the Great Khan of the Mongol Empire, did not go to conquer the Mongols.
That's a good number of counter-examples, yes? At the very least it sure does seem like you need a few qualifiers to your blanket statement.
> I mean this to me just reeks of bias
I mean, your choice of picking Columbus's first voyage just reeks of bias too, so it's getting pretty stinky here.
> and encomienda is just slavery with extra steps
Really? That's the argument you want to make?
The point is that he's unique because he's one who introduced that abominable system to Americas.
Yes, there were other forms of slavery. Then again, there were other explorers. The world was well-versed in exploration before Columbus was even born. If you diminish the uniqueness of Columbus's brutality, you end up also diminishing the uniqueness of his first voyage.
Yes, someone else could have introduced the encomienda system. And someone else would have made the trip across the Atlantic had Columbus not managed it.
And someone else would have been the first person on the Moon had Armstrong caught the flu or been in a bad accident, thus preventing him from flying that mission.
> exploration itself should be celebrated
Because it's called "Columbus Day", not "Exploration Day", and because the holiday started not in celebration of exploration but as a sort of Italian Pride festival to overcome American xenophobia against the newly-arrived non-white/non-Protestant immigrants.
If you're going to rebrand it, that's fine. But since your idea of "explorer" is closely tied to some pretty horrid behavior, I don't think you'll get much support.