Different stable coin issuers means different companies so merging may take a lot of talk and agreements between them its not easy to happen especially when it comes to profit. Much better if they will issue coins separately and have different advantages from each other so investors will have many choice of what they want to use.
merging all stable coins into one coin only really difficult, most of them have different algorithm about pegging their value to dollar. some of them even have value less $1 such as stable coin in wave recently that reach 0,65. personally i see it as difficult thing to unite them.