If you are going to use one strategy then you probably your strategy won't be good for all purposes. That's why for some strategies only work in certain cases and they probably master those cases to make a benefit on their strategy. You'd probably need to at least master one strategy before even bother to try other strategies, sometimes you don't need to learn all of the strategies just to get profit.
Agree with your opinion, we have to have many strategies in trading, because when the first strategy doesn't produce results then what we have to do is change to another strategy so that everything runs smoothly, there are many strategies that we have to learn, especially in trading, if we Being stuck with one strategy will make us overwhelmed when that strategy is not realized, which is why we have to learn a lot more from experience of being directly involved in the field, so that we understand the senior strategies they use so that their market is safe and runs smoothly.