For a lot of people bitcoin is an embodiment of cryptocurrency so whenever btc price changes it pulls almost everything else after it. I believe with time bitcoin's share of the market will get a bit smaller which will give alts a bit more independence.
Its true but when bitcoin loss power to other cyrptocurrency mean bitcoin also will be left by its holder.
Bitcoin holder now still exist because bitcoin is like controller. When price bitcoin is down and its followed by other coin. Its not 100% but its strong control enough.