I think bitcoin would remain pretty much the same thing, Although alt-coins are a plus since it allows more options I doubt it would have a big negative impact if it disappears, The opposite "Alt-coins without bitcoins" is almost impossible to remember since they are even valued in BTC.
i somehow not agree, without alt coin there wont be any alt to btc trading, so the transaction usage is decrease, which is going to cause bitcoin price become smaller, and without alt coin it will be a boring crypto currency world, because we dont have any choice of choosing coin and who knows someday bitcoin is dead then we will still have another coin to use, so alt coin is very useful to bitcoin and to us