I was worried about this too, before I got my Sage 2. Then I came across a book called "the Tactical Folding Knife" by Bob Terzuola, and he's a huge advocate of titanium for liner locks, which would have the same wear issues. He stated that titanium will wear in a bit, and then the contact face becomes work hardened and lockup stays pretty much constant. My Sage 2 is at 50%, and has been there for a while, after much opening and closing.
As far as replacement issues, I've never known Spyderco to not do the right thing. They did have wear issues on the Ti Millie, which they fixed with a steel insert, and as far as I know, customers could exchange their Ti-only one with a steel-insert one. DOn't quote me on that, that's heresay, but it fits with the company's track record. I'm sure if one wore out on you in a short amount of time, it would be replaced. I think the warranty clause is more to keep people from bringing is a 15-year old liner lock that has worn out and asking for a replacement.