Per my understanding, current cryptocurrencies are not suitable for payment since their value is not stable
Wrong. They are suitable for payments, you can actually buy stuff (a house, a t-shirt, digital goods, etc) using cryptocurrencies, though only a handful of cryptos are being accepted by merchants.
Will anybody use bitcoin to pay for a coffee now?
With how cheap the fees have been over the past few weeks (1-10 Sat/b), yes, everyone can pay for a coffe now.
USDT is a solution since it provide a stable value, but it is actually not a decentralized currency.
USDT is a ticking time bomb, I wouldn't count on such a shady cryptocurrency as a a solution for anything.
I think there is a big opportunity to make a cryptocurrency that has a stable value and could be used for payment.
Again, cryptocurrencies are being used as payments by many people around the globe even though they are highly volatile. Also, it would be really difficult to create a "stable" cryptocurrency since, one way or another, it would be part of the high volatility and movements of the crypto market.