if another currency replace bitcoin, after its adoption is complete or in a very good position, many would lose a tons of money, although the transition happen gradually
take a look at the litecoin ,doesn't deem too likely for bitcoin to be ever replaced
Well I don't think litecoin is a good example of a coin that could replace bitcoin, as they're structurally identical.
It would have to be a crypto that really brought something new, either in terms of speed, cost, anonymity, etc. Such currencies exist, but bitcoin has such a huge head start that it would take an exceptional alternative to replace it.