When Bitcoin moves 95% alts dumped. BTC moves up or down, Alts goes down. where we invest and no one can predict the market movement.
No your wrong, look at the price history of bitcoin and alts from January this year bitcoin up and most alts also pump, I dont know where you get this observation btc up alts down, in a bull market when btc up alts also in uptrend look at eth, when btc pump eth also pump much higher than btc this year. Diversification is always the best strategy in investment tried and tested it really works.
Bitcoin is the main trend setter thats why altcoins would most likely be going up or on the same path but it cant really be denied that there are coins which are opposing into the movement.
Diversification is always been recommended if your budget permits you but if not or only limited then its up to your own choice because success in investment will really
vary on how you do handle up yourself on these kind of situations. Diversify while you can but be wise on taking such step.