The key is the price on the chart. A. John W. Henry , for example, has looked back to the 1800's to study market behavior. Markets trended back then too. You can't predict trends, that's why you need a system. A. A trader that uses a constant position size gives up an important edge much in the same way as does a blackjack player who always bets the same regardless of the cards on the table.
Money management dictates how much to buy and sell.
The objective is to make money while controlling downside risk. Simply buying and holding a financial instrument does not satisfy the objective. Buy and hold is lazy trading. With markets going up and down, it doesn't help you to make money on the downside. The upside will take care of itself.