An alternative approach I would like to see realized would be the trading volume to goods and services instead of the trading volume to fiat currencies and cryptocurrencies. For example, one could try to aggregate the information of Bitpay-like payment processors (with CoinPayments in the case of altcoins), OpenBazaar-like decentralized markets and centralized e-commerce sites. That would indicate if a cryptocurrency is really used. And here, I think, Bitcoin would beat all competitors by a very high margin (I wouldn't be surprised if it's over 90 percent)
I guess you still wouldn't be able to get reliable data
Obviously, there are quite a few off-chain transactions which cannot possibly be tracked and taken into account. I've heard that some merchants are accepting only off-chain transactions, for example, from one Coinbase wallet to another such wallet. These transfers are instant and don't require any fees, so their advantage is evident to both buyers and sellers. Though I agree that such info could be very useful in calculating the real value of Bitcoin, i.e. its value as a currency completely stripped of its speculative utility