Investors and nonprofessional people have different thinking about bitcoin. But in my thinking now bitcoin is still worth-able and its price groow above $1000. So we have no argue to discuss about bitcoin price value. In my eyes, bitcoin already achieved a successful place and now it is depend on ourself how we gain and make profit from bitcoin.
Succesfull for a currency in my perspective is not determined on how much the coin's value is, but how's its coverage and usage in this world. It's no use if there's some altcoins which have overwhelming value but only be used among communities which coverage is low. It's ain't real currency but more likely a valuable digital properties.Then all currencies must be not useful then. Never have I seen a currency that's accepted everywhere, even dollars. People say that dollars is accepted everywhere but it is not. Try going abroad which isn't an English speaking country and most places will not accept your precious USD especially if the store is away from the city.
Bitcoin *can* become a currency that's accepted everywhere but we are far from that. So bitcoin might be not useful then according to your statement.
A currency only becomes useless when it doesn't have value anymore. Let's say your government announced that they are abandoning all the currencies that they have issued or your currency has inflated a lot. Your currency will still be accepted but it will have very little value that it has become useless.