We see from FC and NEM that the new Sovereign class can give and take with the some of the fiercest ships in the galaxy. Borg cubes and Scimitar dreadnoughts. Seeing the ENT-E fight an uphill battle against a fleet of Jem'Hadar ships would've been epic to see on the big screen. Lest we not forget the DS9 season 7 episode Penumbra where Weyoun name drops the Son'a, and they were directed to protect a Ketracel White facility. Riker in INS said the Son'a were known manufacturers of the narcotic enzyme. Since the Son'a were apart of the Dominion, why not use the A guard (Jem'Hadar, Vorta, and Changelings) against the ENT-E for a movie? INS seems so phoned in when you think of what could've been.
The producers were deadset against making a 'war movie'. I simply don't see how to make the Dominion the main villains without bringing all that baggage.