Multiclassing. From the very beginning, a Gish has been all about multiclassing, as the term comes from Githyanki Fighter/Mages.
Now back then, multiclassing was pretty good- you got a slow start, but after awhile, you're about a level and a half behind a generalist. Sure, there wasn't any real way to blend your talents (beyond self-buffing, which is generally a bad use of action economy unless you actually have the opportunity to pre-buff), but you could be a slightly worse Wizard with more hit points and fighting ability superior to most other characters as a fallback. Or your first option, however you prefer.
Then for awhile, being a Fighter/Wizard kind of sucked until WotC bent over backwards to create Prestige Class after Prestige Class to let you actually achieve the end goal (Eldritch Knight, Spellsword, Bladesinger, Abjurant Champion, etc. etc.) and even tried their hand at a base class to do the same (the ill-fated Hexblade, and later the Duskblade. I'll count the Swordsage, even though they don't actually cast spells, simply because they allowed you to fulfill the fantasy of teleporting about and attacking with flaming swords very well).
We'll skip 4e because even if you were stuck with the Martial Power Source, you could pick up Rituals and easily pretend you were a magical fighter (without actually playing a class that was supposed to be one). And the hybrid rules let you go a step beyond, especially with hybrid Warlocks thanks to Eldritch Strike.
In 5e though, we run into this very strange situation where every class can be good at fighting and being a Fighter 2/Wizard 5 feels pretty good compared to a 7th level Eldritch Knight. Sure, they can throw a cantrip in with their attack action, but you got Action Surge Fireball a full 6 levels earlier!
Any dedicated "Gish" class is going to run into this same issue. Worse, there's always the possibility unless very carefully designed, multiclassing out of the Gish class into a full caster will eventually be far superior than remaining a Gish until high levels.
Now, if you don't play with multiclassing, that's not a problem, but a lot of people do, and you can't just throw your hands in the air and say "oh you let multiclassing in? not our fault you broke the game!". I mean, WotC tried that in 2014, and look where that got them, lol.