In one of my unused DS9 pitch ideas from years back, I planned to have a scene where a character -- I think it was Dax -- convinced the Prophet to help them or let them know about a problem they needed to address, and afterward she asked, "If you already knew this was going to happen, why did you need me to tell you first?" And they answered, "If you know you will exhale, why do you inhale?" The idea being that even if you know in advance that something will happen, that doesn't mean you can skip over the steps that lead to it.
Yes, all time is one to the Prophets, so they would've known "all along" that they would get rid of the Dominion fleet at Sisko's urging -- but the reason they knew that is because
he urged them at that point in his timeline. It doesn't make sense, therefore, to say that they should've acted without him having to ask, because then he wouldn't have asked and they wouldn't have known to help. The process of cause and effect still had to play out in order to produce the shape of events. The fact that they can see all causes and effects simultaneously doesn't change that.