He could have, in fact that was basically what the Republic fleet had for the Clone Wars. The point of the larger star destroyers was to have better ships than any opponent might have based on the experience from the Clone Wars. Also intimidation factor as these new ships would be used to crush the remains of the Separatists' forces, retake neutral star systems that had attempted to stay out of the fighting by removed themselves from the Republic, and later to crush resistance to Imperial rule. And they are fairly effective at it most of the time. Sometimes a Rebel cell is rather skilled or gets very lucky and takes one Star Destroyer out. It might be the Imperial II class Star Destroyers were built to counter the ways Rebels cells had destroyed the earlier Imperial I class Star Destroyers.
I'm not up to speed on the Expanded Universe (not that it matters anymore with the new material coming out) but it looks more and more like the I-class was actually designed to be less of an effective combatant and more of a terror weapon along the lines of the Death Star. That is, against rebels, insurgents, terrorists, dissidents or common criminals it's the scariest thing in the sky; too fast to outrun and too heavily armed to even THINK about resisting. It carries whole squadrons of TIE fighters and a huge number of troops, so it's pretty much the ultimate badass in any situation you send it into...
UNLESS you're sending it to fight someone with an actual military and actual warships. I keep getting the impression that the I-class is less than ideal in a stand up fight; it's not agile enough to maneuver strategically, and also relatively lightly armed for its size. The swarms of TIE fighters that can strafe the shit out of an unruly population and gun down entire armored columns are pretty much clay pigeons against REAL star fighters and next to useless against warships; same for TIE bombers, which can dish out some punishment but aren't well suited in fleet actions. You get the TIE interceptors later, but those are basically high-grade toys for ace pilots who really know what they're doing and few Imperials are trusted with them because of the risk of defection. In fact the ENTIRE EMPIRE seems to run this war: Storm Trooper armor LOOKS really bad ass, but it offers no protection whatsoever and seriously hinders the wearers hearing and eyesight. TIE fighters LOOK really scary when they're flying right at you, but they have no deflector shields, no real anti-ship weapons, and a cockpit design primarily optimized to give the pilot the best possible view of what he's shooting at and practically nothing else; it's the ideal spacecraft for strafing women and children, just as long as they're not TOO heavily armed.
I think Star Destroyers follow the same basic pattern: they LOOK impressive, and the Empire talks up how badass they are, but they're actually quite ineffectual against all but the weakest opponents. The Star Destroyers are the Super Carriers of the SW galaxy: they're the ideal weapon for terrorist cells of Tatooine, crushing the peasant uprisings on Mandalore and "sending a message" to those obnoxious wookies on the outer rim, but if you put them into a battle against anyone who knows how to fight back, they're just big sitting targets that can barely defend themselves. And again, this seems to be true of the Death Star as well, since both versions of it wound up being destroyed by a little bit of sabotage and some fighter squadrons leading precision strikes. Contrast with, say, a U.S. Navy Carrier group, which for all its destructive power and capacity for military intimidation, would get eaten for breakfast by a skilled captain in a diesel sub. I get the feeling more than a few of the Rebel victories against the Empire have the same dynamic: your big imperial star destroyers can intimidate dissidents and activists, but that a squadron of X-wings that just came out of your blind spot is about to void your warrantee.
Federation starships are not designed with conquest, nor galactic scale warfare in mind. They have no need to carry tens of thousands of soldiers, prefab fortresses, armor columns, and massive amounts of fighters to perform their missions. Federation starships rarely have to travel to the other side of the galaxy, much less in less than a year, to possibly hours or days. Even the Klingons don't design for that as a warrior race. But then it seems like most invasions in Star Trek can be done with only a few thousand troops for an entire planet (which if the planet it reasonably equal in technology, seems a tad under staffed for an invasion of planets with millions to billion of beings on them). The Galactic Empire at least puts down an traditional army so it can take a capital city easily then worry about resistance once they have a foothold and space superiority.
Star Trek also has different constraints as far as combat is concerned. Where the Empire and the Republic both fight for control of entire planets and the adjacent star systems, Star Trek races seem to fight for control of specific REGIONS of planets that have the resources they want. This leads to the tendency of battles to be compressed into "choke points" very close to the objective; starships fight in orbit of contested planets with the goal of preventing their rivals from sending troops and materials to the drop zone, and the troops themselves ignore the entire million-square-kilometer surface of the world to fight over a hundred or so square meters that contain the prize. Where the Republic or the Empire would have to send something like a hundred ships and a hundred thousand troops to try and take a planet the size of Alderaan -- and even with minimal resistance this would be the equivalent of ALL of the allied forces and their operations in all of World War-II compressed into a period of about six months -- Starfleet or the Klingons prefer to restrict their operations to, say, six cruisers or so concentrating on the one region of the planet that has those precious dilithium crystals, or that one shipyard in orbit of the planet, or that long-range communications relay that's been spying on their fleet movements, etc. Invasions of major populated worlds seem to be more rare, so most of the fighting involves battling for control of resources the populations of those worlds care about.
IOW, in the SW universe they fight over territory and volume. In Star Trek, they fight over resources. Resources aren't so spread out as territory, so people who emphasize war as a way of life tend to build smaller and faster ships that can get to those resources more efficiently and kill anyone who tries to get near them.