Make magic a depletable resource.
In one of <PERSON> Canavans series the magic in the world is an energy in the environment around them. A magic user is a person who can take some of this magic out of the surrounding world and convert that energy into something else. Maybe a force that moves something or a light or whatever manifestation of magic is possible in that world.
But as you use energy in a certain area that area becomes more depleted. So in a war zone type scenario each side would have magical shields and be attacking at the same time, rapidly depleting the stores of magical energy in the environment around them. And the strongest magic users are the ones who can take magic from the environment further away. This also adds some tactics as moving around the battlefield gives you access to more un-depleted sources of magic.
In this world the magic in an area 'regenerates' by having the relative gap of low magical energy being filled from the magic around that gap. Like taking a bucket of water out of a pool will cause the hole in the water to be filled from the water around it. 261
But in this world magic fills these gaps a lot slower than water would fill a gap.
You could use this sort of idea in your world. So that as people use magic and convert it from its magical form to something else the total magical energy in the world gets depleted and diluted. If you then need to replenish the magic in the world you can come up with some sort of mechanism to turn other forms of energy (or matter) into magical energy that helps replenish magical energy supplies in the world.
Or make magic users greedy.
This entire scenario assumes that magic in the world is depleting and the end-users (people) are not just becoming unable to access it.
For that sort of scenario I'd make it so that the magic users want their abilities to be exclusive to them only. They don't want common folk to be able to perform the same magics as them or their role in society would change. They'd go from being rich and famous to just being another normal person. It's in their interests to prevent the use of magic from destroying their careers.
If your world is the sort where magic users are common folk, then that scenario wouldn't work. But with that many people using magic all the time I think it's reasonable to think that they'd use up the magical energy present in the world. At a certain point it'd be so diluted that only the most talented magic users can do anything with magic at all. 227