Well, why don't the toads check the summoningscroll. They found out that way that the messenger was killed. Besides that frog oil has high healing capabilities, he was the frog sage so he might have some kind of last minute jutsu. And last reason he might come back, Tsunade placed a bet on him he wouldn't come back. And she always is unlucky. Kishi has plenty of options to make him come back. But if it doesn't happen soon i don't see it happening anymore. Besides you can't keep reviving heroes, Jiraiya put his life on the line he could have fled easily but he found out peins secret. He helped saving Konoha and died like a hero, just like Minato, for Peace.