but it's still possible that a coin like Monero Or Shadowcash couldn't catch a Niche market and hold it even a small percentage of DM transactions going into either coins instead of only Bitcoin could have dramatic effects on price.
In fact, the use as a currency (and ONLY as a currency) has amazingly little effect on market cap. Bitcoin is WAY WAY WAY too high to be driven by "currency demand".
When using Fisher's formula, you essentially have that the market cap of a currency equals the yearly value transacted as currency (the size of its market), divided by the number of times a currency unit changed hands during a year as a currency.
For instance, if on average, a currency unit is kept 2 weeks between obtaining it (in a trade) and spending it (in another trade: pure currency use, no hodling, no speculation, just an intermediate good between the first and the second trade), then this is a factor of 24.
So if the market cap of that currency is, say 100 million $, then it supported a 2.4 billion dollar per year market.
If the trades are much faster, say, 1 day between obtaining the currency (say, on an exchange against fiat) and buying something with it on DM, then this factor rises to 365. That means that with a 100 million $ market cap of the currency, you can handle a 36.5 billion dollar per year market.
In the extreme limit, if you think of bitcoin and you wait only 6 blocks between getting it, and spending it, that amounts to 1 hour of "keeping the coin", then our factor in Fisher's formula becomes 8760. A currency market cap of 100 million $ can then sustain a market of 876 billion dollars a year. That is more than the GDP of some of the smaller European countries.
So depending on the "hold time" of a currency, you do not need huge market caps to sustain a large market.
In principle, with bitcoin's market cap, you could sustain a 87 trillion $ market, which is of the order of the world GDP.... except that bitcoin can't because of the block size limit.