Do you realise that for something to be money or currency (e.g. medium of exchange), it must have
value? How do people know what is the value of 1 BTC or 1 dollar or 1 ounce of gold? They look at how much
other people are willing to pay to
hold it. Why would people hold gold, dollars or bitcoins? Not only in speculation that they will be worth more tomorrow. But also because they are more marketable than groceries and no one know exactly what you want to buy tomorrow. So we want to hold some cash for the short-term and long-term future - we never know what exactly we'd want to buy.
For transactions the exact value does not matter, only matters how liquid this value is. And liquidity is measured by number of people willing to buy that assets at that price. In case of Bitcoin, the more people want to hold it, the higher the price will be (because they need to buy them from someone and outbid others).
Now, imagine Bitcoin and Freicoin. Freicoin loses value ("demurrage") over time. So people would be motivated to hold bitcoin instead. So Bitcoin will gain bigger liquidity and higher value. People will see that Bitcoin is more sellable on the market and thus is wider accepted. So they can rely on Bitcoin's price when doing transactions. While Freicoin remains less liquid and thus less suitable for exchanges.
The currency is the most marketable, most valuable asset. And to make it so it must be the cheapest to store and transport, hardest to produce and impossible to counterfeit.