Richard, thank you for posing your question. MAPI is not going to replace EAS. Exchange ActiveSync is the industry standard for mobile syncing. SmarterMail is continue to support EAS particularly for this purpose. There are certain other clients, for example, Windows Mail app, that also uses EAS.
Though EAS works with Outlook 2013+, Microsoft has made it extremely clear that EAS is NOT technically supported in Outlook. Because many of our customers use EAS to sync with Outlook and in some case extremely large mailboxes, there are issues with Outlook and EAS. MAPI primarily what Exchange uses to communicate with Outlook. Thus SmarterMail is also implementing MAPI for more native communication and more functionality between SmarterMail servers and Outlook.
So, long story short, EAS is not replacing MAPI. They both serve different, but similar purposes.