The problem is people don't want a gish. They want a fighter/mage that is comparable to their single-class fellows. It can't be as good as a wizard and a fighter at the same time. So we need to find the gestalt a bit better than Eldritch Knight (too fighter-y) or Bladesinger (too wizardy).
I was looking through my notes from the beginning of 5E and how to fix the Pact of the Blade Warlock. In 3E, the warlock could convert his Eldritch Blast into an Eldritch Glaive and be all melee with it. The Blade-lock has no incentive to swing his weapon over EB because, mechanically, it is inferior. Why not make a melee-only version of the Eldritch Blast cantrip? Grant the class no extra attack ability, just follow the scaling of cantrips. Or don't make it an actual cantrip but a class ability that scales with class level. No dipping shenanigans. Add some riders, like the invocations for warlocks, call them sword forms, to get caster stat bonus to damage, knock prone, add bonus to AC. Give them 1/2 caster progression and the wizard spell list. Or curate a custom spell list.
So you have a class that can summon a magical blade to fight with like a fighter one round and drop a fireball the next. A true fighter will be a better melee combatant but the spellsword or stabnerd can be a real threat.