I don't totally agree with your logic of selling old coins are buying new coins, which ones are the old coins you are talking about? Bitcoin is the Genesis of all cryptos, Ethereum, BNB, Cardano just to mention a few are some of the old projects and I believe are still some if not the best coins you should be investing in, some argument is just with the sell old coins and increase/add new coins. cos it could be misleading.
But it has to be done to sell old coins.
Just look at Litecoin!
It was very famous but now, nobody wants Litecoin anymore.
New coins have outscored Litecoin and it will happen again.
Right now, Tron, Cardano and Ripple will follow to what happened Litecoin.
Only few coins will keep a position as a top project and a bear market is a good time to buy fresh coins.