Almost true but not completly full. I mean there lot of cryptocurrency you can dirrectly buy with fiat currency for example litecoin, nxt, waves, and xlm. You can dirrectly buy all of those coin. And buy altcoin not useless cause u believe in that coin, and some coin have its own use.
Yes there are many other currencies which one can directly buy from fiat, but what kind of marketcap we get from such pairings. It is just nothing campared to bitcoin and ethereum. Bitcoin and ethereum have become sacrosanct for all the cryptocurrency investors and a lot of new users. The major exchanges binance and bittrex doesn't offer any fiat pairings. The currencies which are tied up with usd and krw also don't provide the volume which a crypto volume provides. Either it is the fear of taxation or something else, but we need to see multiple pairings or a facility where we don't have to go through a mediator like btc/eth and simply buy litecoin from ripple or sell lisk and get ark . There should be no barriers to direct transaction. The volume will get distributed a lot but it will create a lot of ease for people who wish to buy a particular currency from another currency without going through a standard mediator.