It's pretty easy to deny satellite coverage. It's trivial to track satellite locations, and they pass over on a very regular schedule (they can't just up and change orbit on a whim). All you have to do is cover everything sensitive with a tarp at the time they fly over. Anyone can do this, and it was one of the main tactics used by the USSR and China against spy satellites during the cold war.
> and are we really going to start flying hypersonic jets over their airspace anyway?
If you can make it stealthy enough they won't even know ;) And you can also collect a lot of useful intel by flying a plane just outside the border (you can scan inside the border with radar, you can learn about their radar systems by recording all radar frequencies you see, you can record all radio chatter, and so on).
Atmospheric flights definitely still have a role to play in espionage.