The essence of Bitcoin is a mode of payment that seems will be not focused on here. Although you can say that you can use Bitcoin as an asset for the backed assets.
But it's still difficult for now, especially with the volatility and there are still some people who are not ok with using Bitcoin.
Speaking of altcoins, I don't think they are backed with Bitcoin because what can we only see is correlation, like most of altcoins' price action are following with Bitcoin's.