And you deliberately don't want to know since that would punch holes in your faith.
Exactly what do you think I am assuming? Are rocks as hard now as they were a million years ago? You think that is just an assumption? Do you think the laws of physics are the same as they were a million years ago?
Do you think water is somehow different a million years ago?
These are kind of fundamental aspects of science, that water IS the same as it was a million years ago and will be the same a million years from now and the same with the rocks of the GC and the laws of physics.
You just don't want to take the small leap of logic to conclude the GC actually IS millions of years ago except millions of years ago the canyon was not as deep because of the erosion of the rocks by the rushing water.
Did you forget we have photo's of the GC from over 100 years ago showing only a tiny amount of change.
That is solid evidence not guesswork.
And because of that we can with some confidence run the clock backwards to figure out roughly how long ago the GC was at the beginning of it's cutting.