It’s not the amount of time that Spyderco is factoring in, it’s more the cost of materials, man hours, services outside the plant (assuming the part in question is heat treated or anodized,etc.) that go into the process of making the part of the knife you want replaced. It gets expensive just to replace a part that was bought and paid for decades ago. Even though I’m sure Spyderco could easily absorb the loss of a liner or scale (I’m not sure which it is for the Police model) they have to draw a line somewhere or else they get people like that one fella who posted about trying to get a fresh Bali from Benchmade after getting one from his dad that was roughly the same amount of time old.
Also, twenty five years is a lifetime in the manufacturing world. A part could go through a handful of minor tweaks and changes in one year that would make it incompatible with previous incarnations of the knife. Chances are the action would be worse if Spyderco just slapped a new lockbar or blade on the existing knife and sent it back. Then the OP would have a whole different thread about the warranty service.
To the OP, I say twenty five years is a good run. I thinks it’s time to bite the bullet and purchase a new knife, easier to say when it’s someone else’s paycheck though.