So pretty much everyone will have some funds stashed away in a half dozen cryptos, just like you have credit cards, a bank ATM card, paper bills and metal change all filling your wallet (not to mention a checkbook).
O God, that's exactly what we're trying to avoid, aren't we? To have dozens of ways to spend our money, most of which go through banks and other institutions. Also, even if that's the case, my credit card, debit card, banknotes and checks are all in the same currency. If I want to buy chewing gum with my credit card, there's no one to stop me, and there's no hassle about the exchange rate between ChewingCoin and SalaryCoin.