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I don't know what the failure mode is? Is the carbide all the way through? I'm assuming it'll wear through the same as an aluminium rim, in which case I/we should still have many years left, I'm expecting mine to see me out.
It is my understanding that there are bits of carbide impregnated in the aluminum and that carbide is what makes it harder wearing. But I believe that the carbide is not all the way through, only part way. I suspect that the rim has less tensile strength where the carbide particles have been impregnated into the aluminum matrix. Thus I assume that most of the rim is pure aluminum alloy without carbide bits.
If that is correct, it should keep going until on some part(s) of the rim the carbide has worn through and then the aluminum can start to wear down faster in that part(s) of the rim.
The way I look at it is that the rims will last longer than my lifespan, but you are a higher mileage rider than I am and maybe you will need to replace a rim on occasion, but likely years from now.
I would not be surprised if the CSS rims are more likely to fail from a crack or internal corrosion in the hollow space than they are from brake surface wear.
Decades ago I worked with diamond rock core drilling. There were two kinds of drilling bits, surface set and impregnated. And the description of the CSS rims sounds a lot like impregnated diamond drilling bits as a final product. The manufacturing process was quite different, but the end result was similar. Basically a metal matrix holds bits of a very hard substance in place, and that hard substance is what provides the long wearing life span of the surface.
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On normal aluminum rims, I try to keep my brake pads clean. Any bits of metal I will dig out of the pads. And when my pads get a dark gray coating from the rim, that dark gray is aluminum oxide which is a much harder substance than the aluminum in the rim, thus I have sanded down my brake pads to remove that dark gray coating. I do not know if removing that gray coating has helped improve the lifespan of the rim or not, but I still do it on occasion. But I do not get that dark gray on my brake pads or anything else with CSS rims because they do not wear down like aluminum rims.
