I think they are needed for near future but not for too long.
Yeah, it is hard to accomplish adoption without stable coins but for how long ?
I think if we can manage to make some sort of bitcoin or generally crypto payment accepted a lot of places in the world thanks to stablecoins (the buyer can pay with any coin they like but the seller gets stablecoin instead which they can turn into cash) but that should not be too long because there will be a time when banks start using bitcoins in their systems and even store some in their banks which means just like your bank account is your "wallet" in fiat currency it will also become your wallet for your crypto and at that time we will not need stablecoins at all since we will be capable of paying with crypto anywhere we like.