I think you might be missing the assumptions here. No service I've ever seen is 100% available. If you assume that things can (and will!) fail then you need to

I think you might be missing the assumptions here. No service I've ever seen is 100% available. If you assume that things can (and will!) fail then you need to design for failure. Load shedding helps mitigate failures of insufficient capacity. When you need to prioritize some things over others load shedding helps here.