Re: Will stablecoins replace Fiat currencies in the future?
Exactly. So far, stablecoins are used by the minority while that's not the case with Fiat nowadays. Despite crypto's recent popularity, the vast majority of people haven't heard about or even used crypto/Blockchain tech in the first place. It's yet the day where merchants and businesses worldwide are struggling to accept crypto as payment method. The vast majority is with Fiat because of its stability and reliability. Stablecoins are as stable as Fiat, but they come with their own set of problems. As long as Fiat is easier to use than a stablecoin, I don't see stablecoins like USD Coin or Tether replacing traditional Fiat currencies anytime soon.
The minority that used cryptocurrencies will come across stable coins as well but you should not expect that people will use these stable currencies outside of the cryptocurrency space and majority in the cryptocurrency space also have trust issues with these companies that issues these stable currencies as they are not transparent with their financial assets. Do you trust these stable currencies to hold for the long term and if so what are the coins you trust as i would like to know.
Nonetheless, we cannot deny the fact that crypto is rising in popularity at a fast pace. Who knows if someday major retailers being accepting stablecoins as payment method? Even if this happens in the future, stablecoins won't replace Fiat because they're not backed by the government. That could change if governments back an specific stablecoin as a Fiat currency of their own. In the strange and bizarre world of crypto, anything can happen. Be prepared for the worse if governments decide to go rogue against the industry in the future. Just my thoughts 
There is no way major retailers will be accepting stable coins as payment method, either you need to pay extra for the transaction charges every time you purchase something and as a customer how long you will be willing to pay extra for a product.