Sure, the wormhole prophets had been well established in the SERIES.
But during the Dominion War, IIRC they had never been seen or even referred to in any way whatsoever.
But that was exactly why it was valuable to bring them in at this point. The Prophets were a major part of the DS9 backstory. They lived inside the wormhole that gave the Dominion access to the Alpha Quadrant. It would've been a huge oversight and missed opportunity if the series
hadn't involved them in the story sooner or later. And it made sense not to get them involved until we reached a point where the stakes were this high and the crisis depended so heavily on passage through the wormhole. It was logical to involve them at some point, and this was a logical point at which to do it.
I think the writers basically pulled them out of their ass when they realized that "Hey!, there is no way we can plausibly have the Federation/Klingon forces win".
More likely, they deliberately crafted a situation that the Federation and Klingons couldn't win on their own
in order to justify Sisko demanding help from the Prophets. It's a mistake to say this came out of the blue, because the tension between Sisko's duties as an officer and his role as the Emissary had been a developing thread in the series for years. Here, the writers created a situation that was designed to bring that tension to a head -- a situation where what Sisko needed to do and what the Prophets needed him to do came into conflict, and somebody needed to give.
By the way, I've always hoped to do a YouTube video where the Dominion reinforcements are actually wiped out when the Defiant releases a monstrously huge specialized warhead inside the wormhole.
You do realize that would probably destroy or at least severely damage the wormhole, right? It's not just an empty bridge, it's a "foreign country" where powerful beings live. It's rarely good strategy to defeat an enemy by devastating the homeland of one of your most powerful allies (or at least, of neutrals you don't want to get mad at you).
I consider the act Deus Ex because it's basically phenomenally cosmic beings waving a magic wand and solving an unsolvable problem for the protagonists. That's not heroic no matter what way you spin it.
Again: That would only be true if the Prophets had intervened
spontaneously. But that's not the case. It was Sisko's words and choices, Sisko's determination and force of will, that persuaded the Prophets to act against their own wishes. And it was Sisko who made the sacrifice necessary to earn their intervention, who had to pay the price later in the series.
A lot of Trek episodes involve the heroes convincing more powerful beings or forces to take action. Pike and Number One convinced the Talosians to set them free. Kirk's refusal to kill convinced the Metrons to let him and the Gorn go. Kirk and Spock's retrieval of George and Gracie enabled the whales to convince the all-powerful Probe to give up and go away. Picard swallowed his pride and asked Q to save them from the Borg in "Q Who." And so on. Many stories are resolved, not by the heroes taking direct action themselves, but by the heroes persuading others to take the decisive action, or not to take it. But it's still the hero whose words and choices set that outcome into motion, and that's what keeps it from being a deus ex machina.
Sisko being a religious figure to the Prophets, and part Prophet due to his conception. Really doesn't help. It's basically Sisko begging his extended supernatural family for a REALLY BIG FAVOR. He even says he needs a "miracle". By Sisko's admission during his conversation with the Prophets, the Prophets had sent the Bajorans, Orbs, Emissarys, and "encouraged" the Bajorans to create a religion based around the Prophets. Sisko proved the Prophets are very much involved in corporeal matters.
Yes --
Sisko proved it. That's a major step forward in the entire arc of the series, and Sisko brought it about himself.
Not to mention that asking the Prophets for a favor at all was a major step for Sisko
personally. I think the mistake some people are making is in looking at this purely on the level of plot and overlooking the far more important level of character. If there's one thing that Michael Piller and his proteges -- including Ira Steven Behr -- believed above all, it's that the most important question in any story is how it affects the characters. Looked at from that standpoint, a deus ex machina ending would've been one that resolved the plot without forcing any
character to face any real challenges or consequences. But this wasn't that kind of ending. This ending forced Sisko to go to a difficult place personally, to force a change in the status of one of his most basic relationships in the series, and to face painful consequences in the future.
It's amusing how all knowing, all seeing beings as the Prophets can't foresee so many alterations to their own designs.
First off, it's a mistake to call them all-knowing or all-seeing. They just perceive past, present, and future as a simultaneous whole. That's a different perception from ours, but it's not the same as omniscience. Think of it like being at the end of your life and being able to look back on all of it with perfect recall. You could access any moment of your personal experience instantaneously, relive your wedding day and then immediately jump to your third birthday and then to your granddaughter's third birthday. It would feel to you as if they were all happening at the same time. But is that omniscience? No, because you could only perceive those things you personally experienced. You could instantly call up any event from your own life, but you couldn't remember something that happened on the other side of the world on your wedding day, because you weren't there.
Second, the Prophets don't "foresee" anything, because they have no concept of "before." They just
see. You can't define how they perceive and act as long as you're trapped by our concepts of time and causality.
Picard didn't ask Q to destroy the cube/fight his battles for him.
But he did ask Q to
resolve the crisis for him, so that's a meaningless distinction. Deus ex machina is a term of literary criticism, so the relevant considerations are those of plot structure and character dynamics. It doesn't matter whether the particular crisis of the plot involves blowing up an enemy, escaping from an enemy, or getting to a dinner date on time. It's still the defining crisis of the plot, and if it's resolved by random intervention by an outside force, it's a deus ex machina. But if it's resolved by the protagonist
convincing that outside force to intervene, then it isn't.
There is something diminishing about the Federations victory because it's basically "Fuck the Dominion we have GOD on our side".
Again, that would only be true if the Prophets had intervened spontaneously -- and continuously. This was something they did with only the greatest reluctance because Sisko convinced them to face their hypocrisy.
S9 Technical Manual? 
