Apple can get around this by making iMessage available online with the best features only available on an Apple device. Android and Windows users would still be able to participate in iMessage group chats and follow their friends but a native experience, and features like Memoji/Animoji, GIFs, etc would need to happen on device.
Best of luck.
Using Telegram and Signal, one can get a feeling how buggy iMessage really is. Sorry but I cannot see all my contacts using other SMS/MMS/IM options jumping onto iMessage. Especially if the bennies are iPhone only. That is dead out of the gate IMO.