You definitely can invest in USD/Euro. That is what people do on forex...
No, you don't invest in USD/EUR pair, you speculate. Forex trading is a zero sum game. My point is that Bitcoin does not reliably produce wealth, it is a currency, similar to speculating on forex. At some point, user numbers will reach saturation, and Bitcoin will no longer produce any returns (except maybe due to deflation).
So people who bought bitcoins in 2010 and saw 1000% returns aren't allowed to say that they made a good investment? They put money in a commodity (gold 2.0) and they came out with more money through appreciation.. sounds like an investment to me.