Doing TA with altcoins is not good especially if they are very new altcoins or with low trading volume.
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.
I do not think OP is talking about new altcoins because they can be good or deadly. New listed coins can give huge ROI but nearly all if them are Ponzi scheme while investors are losing. That is why using technical analysis for them can not give you a good result. Before trading, you should study and analyze the coin that you want to use to trade before you start trading with it.
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.
There are different types of trading. Some trading do not require fundamental analysis.