Apart from Bitcoin, I have a feeling that these huge market capitalizations are not sustainable. Look at the currencies such as Nemcoin and Stratis. They are having a market cap of $2 billion and $1 billion respectively. Does anyone really believes that these sort of market caps are justified? Even the market caps of Ether and Ripple seems quite inflated.
Of course this is not sustainable. These numbers are highly inflated and can not be taken as real metrics as you have already mentioned.
Example: Make your own altcoin with 1 million coins in total supply. Sell 10% which is 100k coins for $1 each. Multiply that with the other 90% you hold and own yourself. Suddenly the overall marketcap of your altcoin is $1million although it got only bought for $100k!
This happens with so many altcoins and ICOs where a majority of coins is not given out or just locked to inflate the marketcap. It will get interesrting when the big guys start to sell off for whatever reasons.
When you check the transactions per block of all those coins you see how much like of ghost towns they all are!!