The opening made during sinus surgery usually lasts. Your sinuses don't close back up on their own. But "lasting" doesn't mean the disease itself is cured.
If your sinus problems came from ongoing swelling — especially with polyps — you'll still need medicine after surgery. Surgery doesn't remove the tendency to swell.
The best way to think about it: surgery opens the door so your medicine can do its job properly. It's not a cure by itself.