He had a point. Drinking willow-bark tea used to be magic. If we know too much about what it does, in real life we no longer call it magic, and in fiction, we complain it doesn't feel like magic.
But we need consistency and rules to avoid springing surprises on the reader. If the hero can do something in the climax, we need to be warned that it's possible. And to have a good enough grasp of the villain's powers to believe it will work.
Changing the rules on the magic can be very tricky indeed. Especially if you use it to make something easier or possible for the heroes. Like having the hero be told that you must use the Language of Magic, and then being told that no, really, you don't. Much better, even if the seventh book of a series, to figure out some creative use of the rules to let you do what you want.
Then you get the Gandalf or Obi-Wan character. One comic showed a RPG based on The Lord of the Rings and the Gandalf character went oh boy -- followed by "What can you do?" "I can talk to birds" Actually he lit the fire in the snowstorm and a few other things, but he gives very little evidence of magic. Obi-Wan only fought one low-life in a bar, and we're all prepared to see him go up against Darth Vader. That archetype is powerful.