When talking about what classes should exist, it also matters are we talking about potentially adding things to the game as it currently exists, or what classes should exist in the next bigger update (The anniversary update?) In the latter case some of the issues of duplication do not need to exist. For example, if it is felt that there should be a dedicated gish class, then they simply can not have Eldritch Knight subclass at all and have a separate gish class instead.
Personally I feel that class design starts to be bit of a mess at this point, as design paradigms have shifted, so I'd rather look at this in the context of clean slate reboot (still assuming the basic mechanics remain roughly the same and compatible of course.) In such context the role of classes as whole can be reassessed. Some can be merged or removed, new ones can be added.
I am not necessarily opposed of dedicated gish class in such a context, though I feel it should have more thematic focus going for it than a generic multiclass build as one class.