Consider none before have ever experimented with the slow down magic to begin with to discover that it was actually gravity manipulation.
Even as a basic start, when he used it and he collapsed into the ground. The immediate thought for fighters would be. What if I used this club, used this and crashed down unto an opponent? Just that as a start would already make it super deadly and it already conflicts with the initial statement that it's "Slow" magic, when it initially seems to make the user just crash into the ground.
Effectively, it's like watching a bunch of blind people who never understand cause and effect get handed magic and told have at it, and even though it's been used in their history, none of the lessons of the past have been carried forward, so any discovery made now will be oh my god we never thought of it this way.
Even just magic that makes things cold. If they have four seasons in their world, then they already know the deadly effect of winter. Just the ability to make things cold is advantageous in many different ways, whether utility wise, warfare wise or even individual combat wise without even diverting into what people would normally think of. Hey, if you're in combat and sweating, what happens if I freeze your sweat? What about your blood?
Basically. All the mages see that their magic is A. And then forget how to go from A to B and hyper focus on A. It is effectively watching a world where the average IQ is 70.