Valid arguments. The SR rule assumes a random distribution of wins and losses. This is normally a good model, but only a model, and real systems can deviate from it.

Even when the model is correct, we must deal with two facts that appear contradictory to the human mind. The longer you trade, the higher the probability for you to have encountered a bad drawdown at some point. But at any given day the chance of a bad drawdown is exactly the same as the day before. So the day we start a system has no special meaning. The bad drawdown can happen within the first week of trading with the same probability as in a week after 10 years trading the system. It only matters how long you traded, not when you started. Thats why the SR rule applies to trade profits, because for having them you must be already trading for some time during which you're exposed to the drawdown risk. In some situations the SR rule is not obvious:

You got two systems A and B. You invest $1000 in A. After a year, the account is at $2000. Now you stop A, go on a long round-the-world trip, then come back and start B. B has never been traded before. Do you invest the full $2000, or only $1440?

Your aunt has traded system A and has doubled her account to $2000 after a year. Now she gives you the $2000 and suggests that you use it for trading the same system. How much do you invest?

You know that your aunt has traded system A and has doubled her account to $2000 within a year. By chance you also have $2000 on your savings account. You now want to use it and start trading the same system. You have never traded before. Do you invest your $2000, or only $1440?