People trade alts with btc, because BTC is easily available and such exchanges requires no verification to buy alts with bitcoins(like binance,yobit).
So a lot of people will prefer Alt/BTC.
I think it also depends on where you live, if your government doesn't care much about who's doing what with his funds, then trading alts with fiat will be a better option.
But if my government keeps an eye on my funds, then I will prefer buying bitcoins from a local dealer.And then use those bitcoins to buy alts.
I also support the pair ADA / USD. An interesting idea for the author, I think that sooner or later we will come to this
In my country there are already exchanges which have Altcoin/Fiat trading system.Check out koinex.in, and it works flawlessly. I prefer trading alts with fiat over cryptos anytime.
But only if the amount is high, because they charge a fees on withdrawals.If only withdrawals were free too