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That would work if an LED failed to an open circuit like an incandescent bulb, but LEDs rarely fail in this manner.  Under normal conditions an LEDs light output will begin to diminish as it nears the end of its useful life, and this happens with negligible change in current draw.  Since the detection circuit relies on reduced current draw to detect failures, this wouldn't work.  Another common failure is driving them with too high a voltage, and in this case they burn up and turn into a closed circuit, which would cause a short and possibly burn up the flasher relay or blow a fuse.  This failure is also not detected by a relay modified with new shunt resistors, since again, that relies on decreased current draw.