I've read that iMessage will support RCS later this year. Don't know if it will come true, though.
I think there’s a misunderstanding with this statement. iMessage will not support RCS. The iPhones will support RCS - it will exist separately from iMessage. The green bubble will still exist but you get to send full or bigger sized media files, see read receipts and you will know when someone else is typing a response. Apple is currently only considering the Universal Profile so there is no end to end encryption yet but that might change.
However iMessage will exist in a bubble of its own.