> Nobody replaces their Mach 3 cartridge after every other shave but that's a common recommendation for DE blades

No, it's not. If anything there is a belief that DE blades actually stay sharp and last longer than cartridge blades. I don't know if there's any evidence to that, but the effect is that many DE shavers keep their blades for a number of shaves. I suspect that some are more inclined to swap blades frequently because the blades are so (comparatively) cheap.