Did that king say this was the 18th summoning they did? So they are just kidnapping people to throw into a war and their only way home is to defeat the enemy? How. Very. Convenient. For. Them. Since this isn't one of those stories where the heros are placed in slave collars I'd say there is nothing stopping them from running off to do their own thing. Why is only the MC getting to do his own thing. What's stopping the rest from using their OP powers to just leave the kingdom? Would you honestly believe a word a kidnapper says?