Something is preventing Starfleet from existing in decisive strength.
Just this, while never specified on screen, something somewhere is creating a production bottleneck.
Four engine pods, more speed or power.
Has it ever been established that more nacelles = more speed/power? Putting more wheels on my car won't make it go any faster
But having four wheel drive does offer advantages over two wheel drive in some situations. Tractor-trailers have eight drive wheels, the Stargazer's four nacelles offer abilities over only two double sized nacelles.
Maybe the "under powered" statement stems from the warp core only being powerful enough to run 2 nacelles at a time.
The Stargazer could have been considered high powered when launched, and Picard was simply commenting on the aging vessel's approaching obsolesce. Comparing it to then "modern" starships.
Even in the TNG end episode the future D had a third nacelle. Why? A dingy medical ship was able to achieve Warp 13 just as well and it only had 2 nacelles..
Perhaps just that, it was "dingy."
If you wanted more power with exist design nacelles (analogy to tires) , merely installing a larger warpcore and feeding the resulting power to two nacelles would only allow you to go so fast, accelerate so fast, and then the nacelles simply would have nothing else to give. The nacelles on the Stargazer would visually appear to be "standard sized" to the ones on the Enterprise refit. The four nacelle design would give twice the destination, twice the warp coils for any additionally produced warp plasma. What would be a dangerous red-line for two nacelles, would be standard operating parameters with four. Higher cruising speed, more acceleration, increased towing, maybe even tighter turns in combat.
On heavy duty pickup trucks, with large tows, you have to put addition drive tires outside of the original two, otherwise when you try to move, two tires will just spin on the pavement, while four will move the load. Doesn't make any difference how big your engine is. (Found this out the hard way)
Race cars have unusually wide drive tires, again to handle the power of the engine.
And even then, I very much have my doubts about long construction times regardless of how new the ship is.
From the scene at the end of Nemesis, where the new bow is being built on the Enterprise, they do indeed build their starships. A Galaxy will take longer than a runabout. It isn't manufactured/replicated already in one piece.
