The phrase buy the dip is only a good advice when you apply it properly by buying the very good altcoins whose value is only down because of the bearish market condition. Because it is not all coins that dip that would recover their value.
For example, early this year (when the market was super bullish) almost all the projects released saw good increase in their value. but that doesn't mean it is all of them that were very good projects. After the market dip, most of them never recovered their value. The crypto market is filled with a lot of scrap altcoins and the market way of sheding them off is to dip (retracement). So it would be in your best interest to only buy very good coins in the market dip.
yes I agree that not many altcoins can rise again after a price drop in the market. I've experienced it myself and it's a mistake I made at that time so I've lost a lot of money. so until now I have never bought an altcoin that doesn't have a great future. for now I only collect coins like ethereum and binance, because both of them I think are able to survive and go back up after a price decline on the market exchange.