Columbus and his voyages are the triggering events of the American colonial period with out a doubt.
But the old world would have discovered the New World eventually. The only questions are; by who and when.
In my opinion for the native Americans things could turned out much worse.
Had the colonization of the Americas been accomplished by the Islamic countries of Turkey and the North African nations I think the outcome could have been much worse.
Lets be real. The Americas being conquered by the Christian countries of Europe is probably one of the least catastrophic outcomes. No matter which culture conquered the Americas the native people were going to be displaced and assimilated in to a more advanced culture.
It is an inescapable lesson of history that a primitive culture when exposed to a more advanced and more aggressive culture will be subsumed in to that culture
All in all things could have been worse.
Disease was the biggest killer, and, as you said, sooner or later the Europeans were going to arrive. Without resistance to those diseases from Europe and Asia the Native Americans were in serious trouble, even if the Europeans had arrived with the most peaceful of intentions.