New altcoins will not have any historic data to look back on, so TA may not work accurately, but that does not mean that you should ignore it completely and use your TA from studying BTC movement to predict the movement of the altcoin.

If you suggest focusing on only BTC price movement to make predictions for the altcoin, it is almost the same as saying someone should focus on only technical analysis and not pay any attention to fundamental analysis, but in doing so, accuracy of analysis is reduced.