Yes, I believe one trading strategy is enough if a trader masters it fully, but that strategy won't work on all market conditions that's another fact and such a trader should understand that thing.

I know many traders who still follow Bollinger bands and Rsi strategy, personally I also somehow follow that strategy from a long time and so far it has given me good results.

And, there's group of trader who believe in buying the dip and holding it for sometime and make profit from the holding, I also follow that strategy and that one has also been really profitable for me.

Those two strategies can be combined into one strategy and they'll still work because when market is dumped then surely RSI and Bollinger Bands strategy also get fulfilled and combining those two results in even better profits if trader is emotionally strong.