I think that cryptocurrencies are more complementary than alternative to each other (although everything is both a little bit complementary and a little bit alternative to everything else).

The more people discover the existence of Bitcoin, the better for Zcash. The more different applications get built on top of Ethereum, the better for Zcash. The more usage Monero gets, the better for Zcash. And vice versa.

One way to think about this that all cryptocurrencies, including Bitcoin are tiny. Bitcoin's market cap of $15B is tiny. Bitcoin's user base — around 20M maybe? tiny tiny tiny Barely enough to sustain a small industry. Maybe too little to sustain a small industry. And that's to say nothing about the other non-Bitcoin cryptocurrencies, which are all clearly too small at the moment to sustain any industry.

Compare Bitcoin's market cap of $15B to gold (around $10T), or fiat currency (around $30T, not counting loans, certificates of deposit, bonds, etc. etc. etc. Just counting coins, paper cash, and checking accounts). http://money.visualcapitalist.com/all-of-the-worlds-money-and-markets-in-one-visualization/

So if Zcash grew to ⅒ of cryptocurrencies, and Bitcoin shrank to ⅒ of cryptocurrencies, at the same time that cryptocurrencies grew to be ⅒ of the gold market, then Bitcoin and Zcash would each be worth about $5000 per coin. Along with eight other cryptocurrencies also worth $5000 per coin.

Bitcoin's user base — maybe (?) somewhere around 10–30M. Compare to the number of holders of credit cards — about 1.5B. Paypal has about 179M active accounts.

Or if you want to compare to a product that is similarly new as Bitcoin is, look at Apple Pay, which is about two years old: about 12M users (active each month).

Let's face the harsh truth: a very plausible future outcome is that all cryptocurrencies dwindle into irrelevance. The best way to avoid that outcome and to maximize the chances of the opposite outcome — that cryptocurrencies become the fabric of the global economy and global society, and become worth hundreds of trillions or quadrillions of dollars — is to focus on creating value for mainstream users instead of focusing on competing with other tiny, fragile, low-value "competitors".