I kind of agree, and kind of disagree.
Bitcoin was the first (2009) and is the best known cryptocurrency. Shortly thereafter (2011) came Litecoin and Namecoin (Swiftcoin as well, now inactive). There are now, per coinmarketcap.com, 1,219 cryptocurrencies currently in circulation. Collectively, they rely on relatively few hash algorithms which underpins the currency.
At present, Bitcoin (cap = $96,193,147,424) and Etherium (cap = $28,348,688,618) dominate the market for cryptocurrencies, 490 cryptocurrencies have a market cap under $100,000, 220 cryptocurrencies have a market cap over $10,000,000 and only 60 have a market cap over $100,000,000. Total market cap for all 1,219 cryptocurrencies is $169,322,974,318 (Oct 27, 8:00 PM UTC) which includes 274 currencies with unknown market cap due to the fact that there are an unknown number of units in circulation.
Similar to gold, there is zero productive capability, but unlike gold, gems and the like, there is nothing tangible with a cryptocurrency. However, there are 162 exchanges that trade one or more cryptocurrencies but, only 10 of them report 24 hour volume over $1,000,000.
No one is making new gold. Unlike a cryptocurrency, the amount available is unknown. While each cryptocurrency has an upper limit to the number of units which can be mined, the number of cryptocurrencies that can be created is unlimited.
In that none of these are backed by anything tangible and virtually anyone (with the knowledge and appropriate computing equipment) can create a new cryptocurrency I am of the opinion that each new currency cheapens all of them.