Well, that reasoning is kind of depressing, actually. It seems to imply that late-24th century Starfleet has transitioned away from an exploratory mission toward being a more strictly military fleet. Seems kind of at odds with the underlying philosophy that always made Trek special.
Well, keep in mind that in the late 24th century, Starfleet almost lost Earth to the Borg -- the deadliest threat they'd ever faced -- and fought a war with the Cardassians, fought a brief war with the Klingons, and the ended up fighting a massive and costly war against the Dominion/Cardassian/Breen alliance. It only makes sense that Starfleet would be thinking in terms of making new ships as rugged and powerful as possible.
However, that does not mean that it was to the exclusion of their exploratory functions. A ship the size of the Enterprise could be outfitted with massive armaments and still have all the science labs, research facilities, diplomatic facilities, and so forth that were necessary to facilitate such missions.
But when push comes to shove, Starfleet's first job is to protect the Federation from attack. All else becomes secondary.