You are correct, in the sense that simply missing in combat is potentially life-threatening fumble as it allows an opponent to act for longer.
The corollary to that is that in dangerous situations, magic simply not being a surefire thing is sufficient to introduce much-desired risk into the game.
There are only two major reasons to add fumbles, or any other carrier effects, on failures:
1) to speed up play by preventing long chains of "nothing happens".
2) to create situations which wouldn't otherwise emerge from the game rules. For example, in combat, you don't need a fumble for the Fighter to drop their weapon, because a miss gives their enemy an opportunity to disarm them, creating the same result. But if swinging your sword at a wall, the wall is not legitimate user of "disarm" action, so under the simplest rules your sword could never be broken or fly off your hand. So a new rule needs to be added if it is acknowledged and desired that yes, if you swing your sword at a wall long enough and hard enough, something might happen.