Or, Starfleet could just build more Defiant and Prometheus class warships while giving older designs better weapons. That's the most practical option.
I would think that for all the construction work and material it would take to build a weaponized replacement saucer for a Galaxy-class starship, Starfleet could build at least half-dozen Defiant-class escorts (each with ablative armor all around, plus each having their own warp drive to boot) and need less personnel to fly them and maintain them. The Galaxy-class ship (unmodified) could be useful in hauling personnel in large numbers to a trouble situation, while the Defiant squadron could handle any confrontations.
Why bother maintaining an extra saucer? The Defiant-class ships could always be docked or used as tactical training ships when not needed for combat.
That would seem to be a more logical and practical approach for Starfleet to take.