Bitcoin is not useless, if it is useless as you said then why people investing in it would have stop. And diverting into other coin is a good idea as other coins are not entirely useless.
Altcoin is independent, meaning it does not depend on bitcoin. Because in cryptocurrency there are thousands of crypto currencies, only the most famous is bitcoin because bitcoin is the first, the price is very expensive and most famous. Even many people don't know crypto but know bitcoin. So, without bitcoin cryptocurrency can still exist because they are independent not related to bitcoin. Because their use and purpose are also different.