My headcanon is that the Zheng He type is Starfleet's primary tactical starship circa 2399, and deployed in force as a response fleet to confront potential hostile armadas. Yes, some designs like the Akira, Sovereign, and other late TNG-era designs are still around, but those are dedicated to other tasks. Because I'm quite sure that Starfleet learned the follies of having Mirandas and Excelsiors still in heavy conflict. Starfleet had learned that the hard way starting with Wolf 359 and especially with the Dominion War.
In Space, having multiple starship types may be aesthetically appealing, but may not be as tactically or strategically effective. With a fleet of largely one design plus variants, the ships may be better at coordinating formations, fighting cohesively, and working together. In other places, I have mentioned the example of the Standard Type battleships, which despite increasing armaments, were very similar in appearance, and also had similar handling characteristics. As well, during the age of sail, often the most effective fleet were those comprising dozens of similar ships, most notably the 74-gun which was well regarded for balance speed, firepower and cost.
In a confrontation like in the finale, it may be more tactically sound to have a fleet of Starfleet's most powerful, toughest tactical starship, as that means you are sending in a force with maximum firepower, manoeuvrability, and durability. No more sending spontaneously-combusting Mirandas and Excelsiors and other kitbashes in small waves. Instead, a whole bunch of heavily armoured starships armed to the teeth warping in en mass.
Vessels like the Defiant, Akira, Yeager may be used for general-purpose defence, but might not hold up when dealing with a massive enemy fleet. But perhaps, the Zheng He is more suited for fleet engagements, and act as the 24th/25th century version of the ship of the line. Meanwhile, older designs are elsewhere continuing their exploration and scientific missions.
TL, DR: The Zheng He-types are designed primarily to serve in tactical fleet operations like a classic ship of the line.
I see it as more of a reflection of the modern day Naval Ship design.
BattleShips / BattleCruisers are obsolete.
The Cruiser is the king of Ship Types and given the advantage of FirePower and Modern Weaponry, Cruisers offer that balance of Size, Power, Defense, and Quantity.