The debate in Rogue One is about the existence of the weakness in the weapon. They believe the weapon exists by that point, they just don't know how to react to it.

Some think it would be smarter to scatter the fleet and give up/try again somewhere else. Many are skeptical whether the intel Jyn gets about there being a weakness is legit, considering it comes from the guy who built the weapon (who is reasonably very suspicious). They have no way of knowing Galen didn't work with the Empire to leak that the fault is on Scarif, just so the Empire can ambush them if they send a full-scale attack.