"Prediction is very difficult, especially if it's about the future."
With that caveat, here are some observations that I can make based on what has already happened over the past year or two:
1/ Email marketing will become increasingly difficult due to Google and others implementing separate folders for "promotional" content - see http://sethgodin.typepad.com/seths_blog/2018/01/an-open-note-to-google.html
2/ GDPR regulation (with effect from May 2018) will restrict the activities of email marketers in the EU region (just Google GDPR if you don't already know about this)
3/ Despite the above, email will not go away. Facebook Messenger and SMS are both avenues that will continue to grow, but email marketing will continue to be profitable for those that invest the time and effort into delivering value to their audience through this medium