Has it not occurred to you yet that all the other coins have their value in BTC? Your statement is absurd.
Let me give you an example. There are hundreds of currencies in the world today. Most of them are worthless garbage, and one think they have in common that each currency has its value denominated in USD.
BTC is to altcoins, what USD is to fiat currencies. Capish?
Your comparison is valid. But if you check currency reserves of central banks, you will notice that the share of USD in their reserves has been declining over the past few years. BRICS countries in particular have been striking deals to settle accounts in their own currencies without USD as a middleman. They are gradually moving away from USD. Nothing is static and nothing is set in stone, but some processes are gradual and may look as if they are not happening if you don't look at longer timeframes.
Same is taking place in crypto currencies. There are people who don't value BTC as much as other cryptos. The number of them is growing. But in crypto currencies everything moves faster. And if you don't invest your BTC, there is no US military to prop up its value for a long time.