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TinyMCE 4.6 dropped support for browsers that Joomla is still supporting; a review of that same changelog document would have shown that. Therefore, Joomla cannot upgrade the distributed TinyMCE editor without abandoning browser support.
Gutenberg is Automattic's attempt to use the open source project "founded" by their CEO to compete with other SaaS solutions by introducing an entirely new management workflow. It has nothing to do with upgrading editors.