I don’t think that is what will happen at all. My interpretation of the law is that a given service must define an API/means to allow other services to insert messages into their network. So that the hard work is on any other app that wants in. Apple will be required to write some code to allow others, say Google, to insert messages, but Apple will not be required to write the code Google needs to achieve this–Google will need to write that code.
To me, the odds are, if everyone has a means to insert messages into other networks, most messaging apps will not bother actually doing it, because it still waters down their product. So they may all open up to others, but find there are no others who care enough to bother. There will probably be some 3rd party, like Pidgin, who will rejoice, and will be the one product that talks to everyone, but virtually no one will use that product.
It COULD be that someone like Microsoft is the big winner here. They don’t really have a winning platform that everyone uses, but they could make their desktop more appealing if it can interoperate with mobile platforms.
Hard to predict what will actually happen but, to me, this whole thing is likely to be a political win that wins the average user exactly nothing.