the prices of altcoins depend on the bitcoin price, because the financial flows constantly flow from one crypto currency to another. naturally, not all are synchronous in their behavior on the market, many assets may not manifest themselves as the trader expects at the moment when the rest behave quite predictably. so you can hold your coins for several years, but it is not known how the market will behave.
The influence of bitcoin is very strong because altcoins is always trading with bitcoin, this is what makes the price of altcoins set in bitcoin or ethereum so that when bitcoin prices drop then altcoins prices also drop.
Of course bitcoin has a very strong influence in determining the future and market of cyrptocurrencies, but I am sure that one day the market can be stable because it can be used for direct transactions.