XEP-136 is a good example why it's a bad idea to squeeze everything into a single specification (or XEP in this case): The reason it was not implemented by most XMPP stacks was, because it's so much you have to implement. XEP-313 focuses on what is important to achieve WhatsApp like persistent chats in XMPP. Functionality which is lacking in XEP-313 can be specified later on in a different XEP. After all that's why it is called the "Extensible Message and Presence Protocol (XMPP)".