Domination will still not go down, and alt will still face a dramatic ratio of low-scale movements with a downward trend very often. if the investor you are referring to is a trader, that makes sense. But for investors who still care about the new shitcoin will continue to exist. I highlight the pressure on altcoin and this still applies to everything because of bitcoin, but it hasn't had a big influence to make many altcoins knocked out, they just go down because of market sentiment.
Bitcoin is the parent of all cryptocurrency currencies currently on the market, so that every bitcoin move will certainly affect the altcoin movement underneath, I think this is due to the BTC trading pair at every exchange, so the influence of Bitcoin on Altcoins will always be there. So, the correlation is only centered when people start taking profits or cutting losses in Bitcoin, by choosing Altcoins or Fiat currencies.
Of course, it is not only that which influences altcoin movements in the exchange but as you said if the market sentiment, both positive and negative will certainly also have a strong impetus for every altcoin and bitcoin price movement in the market.