This question is specifically for the module "Rescue at Rivenroar", but it's applicable to any dungeon-crawl with the following premise - the antagonists have just attacked a village/town, and have withdrew to their stronghold with hostages and treasures. The townfolks hire the PCs to enter the stronghold (a dungeon in this case), and rescue the hostage and retrieve the treasures.
As my groups tend to have very pragmatic players, they will definitely assess the risk of the venture. Off the bat, it looks unreasonable for the following reasons:
The hobgoblins camped at Rivenroar has just attacked a town with 200-men strong garrison, and it is said that the town can muster another 200 if needed.
If the town defenders can fight off a hobgoblin attack with their 200 strong army, why are the PCs required? Since the defenders are able to drive the hobgoblins away, they should have no problem marching in to get rid of those hobgoblins
If the town is unable to clear out the hobgoblins at the Rivenroar crypts, due to a number of problems, how can one expect the PCs to succeed?
Likewise, if the town barely defended itself from the hobgoblins, then the hobgoblins are necessary stronger. Wouldn't sending the PCs to their strong to save the townsfolk and recapture the leaders a foolhardy and suicidal enterprise?
The module for Rescue at Rivenroar doesn't address those questions. So the question is - how can I tell the PCs that it is plausible for them to enter the dungeon and succeeded, considering the odds and that their services are actually needed?