Without bitcoin, the future would simply be private blockchains run by private institutions on the set of trusted servers that they decide. So instead of trustlessness, we'll be offered distributed-trust or something.
It is not feasible for another network, code-base or user-base to simply replace and start doing what bitcoin has been doing for almost 10 years now. Bitcoin is also the most held and most distributed crypto out there. It's the settlement currency of cryptocurrencies. Imagining a crypto market without bitcoin wouldn't be possible as all those people wouldn't simply stop using it altogether.
There was bitcoin without other cryptos and it can still exist on its own. On the other hand, there can be no crypto ecosystem without bitcoin.