I don't see Bitcoin as much as a competitor for gold than for Western Union. Remittances is, as of now, the field where Bitcoin really excels: The volatility is almost non-relevant - even in bear markets - as Bitcoin remittances take place in minutes to a few hours, and there are no international barriers (apart from prohibitions in, at most, a handful countries).

According to this article the yearly remittance volume is 500 billion USD. Bitcoin could capture at least half of it.

Obviously, it isn't that easy to say "now, Bitcoin can have a 250 billion market cap" (only taking into account remittances). An estimation would probably be based on the Quantity Theory of Money. But I estimate that there is a potential for Bitcoin's price to rise at least five times higher than today only if it is able to capture this market.