You make a good point. Without additional safeguards/depending on implementation, W=1 can indeed mean "durability of one".
This also depends on your failure model. If your node crashed (RAM cleared) and your durable storage broke, you're in trouble. If the data was durably persisted and you just have to restart the node, it's a better situation.
This also depends on your failure model. If your node crashed (RAM cleared) and your durable storage broke, you're in trouble. If the data was durably persisted and you just have to restart the node, it's a better situation.