what if US Government will create Mericacoin and advertise it everywhere as "the only reliable cryptocurrency, backed by the economy of USA" ?
They do the same with dollar, same thing as every country does. They give fake promises and make people believe that currencies that we have nowadays are backed by something big and reliable. But, in fact, all currencies are bubbles, inflated only by the trust of people in them.
The problem is, the most part of population will switch to Mericacoin and won't even try other cryptos. Even if the government tell them that it will keep their private keys for some serious reason, such as safety or, let's use the most popular reason - to fight terrorism somehow.
The only way to avoid such situation - to make everyone know that the main thing cryptocurrencies bring - total financial freedom, where your money are truly your's and no government, no department or service can touch them, block your accounts, take part of it to make you pay your taxes and so on.