A keystone is that. Keystones are key, take the keystone out of an arch and the arch collapses. Doesn't that make a keystone "important, salient"? I think it does. Both of those definitions support these words being synonyms. I quoted both definitions because I believe both support my point. (And I quoted both for 'keystone' because if I didn't, I figured you'd disingenuously attribute that omission to malfeasance.)
> As another poster has asked, why would you choose to change a shorter, more common word for a longer, less common one if they mean exactly the same?
My response to him applies to you as well. You would not be nitpicking such word choices if you were not looking for a crack to gain leverage in the conversation.