For all we know, they could have sent the majority of their forces with but a minor contingent remaining behind in the GQ.
That wouldn't alter the balance of power much, though - because the isolated forces on the Alpha side were denied of shipbuilding resources many times over (the first day of the war saw Starfleet destroying several shipyards) and still apparently managed to outproduce the combined Alpha powers! If a beachhead force with portable factories could achieve that, then the main body of Dominion back home in Gamma could have built a force to dwarf all Alpha navies in a forthnight...
The Federation alone should have had more than enough ships to deal with the Dominion. Drama on the other hand dictated otherwise.
Why "should"? What's objectionable about the concept of the Dominion sending through the wormhole as many ships as Starfleet possessed? Even if we're speaking about, say, 10,000 ships, it's just ten thousand-ship convoys, or a hundred hundred-ship ones. We supposedly saw something like a hundred ships push through in "The Die is Cast" (counting the number of quantum singularities on the computer screen), and plenty of Skreean ships came through in "Sanctuary", too.
...a Cardassian EMPIRE (which already had pre-established industry and facilities in the AQ which the Dominon simply took over)
OTOH, that empire had been a pushover in the last conflict with the Federation (in the sense that we barely noticed there was a war going on during TNG), and had only barely held against the Klingons. I wonder whether its war materiel production could have contributed to all that much in the end.
I doubt the Dominion would have been so fortunate if the fight was happening in the GQ.
We never heard of the Alphans replenishing their forces even when the fighting took place on their own turf. The only ship built during the war appeared to be the
Sao Paulo!
70 years down the line, a severely outdated Dominion fleet would reach the AQ, and do what exactly?
The thing it did in the first place - surprise the Feds by alien innovations. After all, the Feds, too, would lack intelligence on their opponent.
And sending one fleet over would allow the Dominion to build a new one the next decade, and a new one the following decade, and to keep on producing newer and perhaps faster fleets so that 70 years down the line, seven overwhelmingly sized invasion forces would arrive simultaneously. It's not as if we ever heard of the Dominion military having anything better to do in the Gamma Quadrant, after all.
What we saw in DS9 gave us evidence of a fanatical breed of oppressors and conquerors, not particularly lacking in means. The Alphans did little to erode that drive or those means, and in turn demonstrated that they weren't particularly willing to reverse the tables and commit to the annihilation of the Dominion. They did demonstrate they were capable of underhanded tricks, though, thereby probably signing their death warrants because there was no way the Dominion could leave them alive now...
Timo Saloniemi