There are lots of coins this days and more are still emerging by the day. If eventually cryptocurrency becomes accepted globally and it's been adopted for usage, do you think all the coins would be accepted for usage or only a selected few?
Look at what big businesses are supporting.
Most of crypto adoption is happening through bitpay. And they only support Bitcoin and BCH.
If they don't care about other coins, they must be sure they're not losing business because of it.
This is why I don't believe that merchant solutions will allow for a multitude of cryptos.
Even if they expand to Ether, I'd say 10 coins is the top.