One part of the idea of the balance between wizards and fighters was, once upon a time, twofold:
- Wizards were not guaranteed to be able to access the spells they wanted. Rolls to learn and costs of spellbooks and no "automatic spell learning" meant that spells were a reward, a chance to do something cool (or not). This lessened over time as players fairly rightly rallied
- Fighters were able to use the most diverse array of weapons, armor, and equipment, which helped them achieve the edge when getting awarded treasure. Who can use the magic sword or the flying shield or the plate armor made from dragon bones? Fighters. No one else (well, sometimes clerics for armor). Over time, the game tried to equalize this aspect of play, giving all characters magic items, and regulating them to ensure a level of balance.
This, combined, meant wizards might not rule the planes or control undead armies, and fighters very well might be able to teleport between planes and fly and turn invisible (due to magic equipment). It certainly didn't hold very tightly like that, but I believe that was part of the original idea of how they would be balanced.
In 1E/2E days, any damage a wizard took while casting a spell would negate the spell as well. So, if the bad guys were reasonably smart, they could have several lower level "minion" types that could hold their action to attack the magic user when they went to cast their spell.