In my opinion, the "monopolistic" aspect of it comes down to the fact that they tied it into an otherwise open messaging system - SMS. You cannot separate SMS messages from iMessages (to my knowledge). So, the only way to know a message was sent via SMS is the green background for incoming messages and the green background plus the "sent via SMS" for outgoing messages. This creates a disdain for SMS, and anyone who uses it over iMessage. It is such a strong feeling, that having green messages makes you "uncool", especially in the younger crowd.
On the other hand, I think the long-term sequalae of the blue-green message is to push people to use stand-alone apps like WhatsApp and FB Messenger. I think it'll be a hard sell at this point to convince a jury that iMessage is an overt monopoly.
The main question I want addressed is: If SMS messages can be directly shown in iMessage, and are not secure, then the argument of not allowing "insecure" 3rd-parties to integrate with iMessage goes out the window. All I want is Android messages to be shown in iMessage. Sure we can make them green, but at least they will be sent over the data network and not SMS.