Good point, many times people think of diversification only in terms of their portfolio, but it is also important to diversify our income streams, many people depend exclusively on their jobs to get some money, but regardless of the amount they earn if at some point they are fired they will find themselves facing huge economic issues really quickly, however if instead we have two or more income streams if one of them dries up we have another ready to help us out soften the blow, until we can find yet another income stream to replace the one we lost.
But there is a catch in crypto trading, alt coins are strictly dependent on BTC for there price gain or loss. Even if we diversify our portfolio, if btc falls all our coins will fall in price. So there is not much gain in diversifying your crypto portfolio. These are just my few satoshis.
I get your point, of course, altcoins will also fall especially during the bear market, but when it comes to investment, we can invest in long term so the short term price volatility would not affect us. Once the market recovers and starts to be bullish again, altcoins will probably be benefited more as they are more profitable than bitcoin, and of course, we can verify that by looking at the top gainers this year so far and comparing it with bitcoin's total gain in percentage.