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.
even I think it's impossible to realize. it's not just from different companies, of course, everyone in the company talks about how stable coins they make will be profitable for them too. many choices and leave it to the user. because everyone has their favorite. no problem with stable coins, they also have their market support.