So a big thing that is discussed when it comes to the Imperial starfleet is that it is entirely entrenched in the Tarkin doctrine. It is said that the entire fleet is based around overwhelming size and firepower to intimidate and demoralize any potential rebellion. The resulting affect on the navy is a fleet of oversized warships that are too inflexible for the war they found themselves in. While I would disagree with the consensus that says: "If the Empire used TIE Defenders, divested the Imperial class, reinstated the Venator, then they would have won!" I do so from the point of view that the Empire lost because they are fascist, and not because of their navy. I am putting forward the idea that the Empire's ships are what inspired the Tarkin doctrine, not the doctrine dictating the ships. While I think this is the case for most of the Empire's ships, it is not the case for all of the Imperial vessels. The Executor class, Death Star, and other overambitious "wunderwaffe" are a result of the Tarkin doctrine, whereas the ISD, and TIE fighter are not.

To start we must first consider what the Imperial navy after the clone wars is comprised of. It has Venators, Acclamators, Victories, dreadnaught cruisers, Arquitens, a small number of Imperators (soon to be renamed Imperials), a handful of Mandators, and numerous other cruiser and frigate classes. You may notice that I mentioned the Imperator/Imperial class, this is because this class of ship was in design during the clone wars and only entered service at the very end of the war. This is a diverse and effective set of warships that all cover each other's gaps, and is quite solid overall.

What is important to consider here is that this fleet already has a few staples of the Imperial starfleet, notably: Imperators, Victories, Arquetins, Gozantis, and small frigates. This means that these ships cannot be a product of the Tarkin doctrine, because they were being designed and built during the Clone wars. The most apparent example is the ISD itself, which I will discuss in greater depth later, as it deserves its own focus. The Victory class is an often overlooked ship, but it did serve well during the clone wars. It was designed in parallel with the Venator, yet due to a long design period, it was introduced after the Venator. It's a decent ship, albeit slow. The Victory II still had similar issues with speed, and it was never as fast as some of its contemporaries.

The small cruisers/frigates are the workhorses of any fleet and it should come as no surprise that they still served within the Imperial navy. They are decent and versatile ships, serving in convoy escort, trade interdiction, and fleet air defence roles. They honestly don't need much work on them, so they simply got upgraded drive systems, reactors, weapon systems, and improved shield generators for their service in the Imperial fleet. No one ever mentions these ships, despite them being very common into the Imperial era.

The TIE fighter is the Empire's fighter of choice, and although I disagree with its flanderization in the community (it trades relatively well with Alliance craft that are not piloted by heroes), it is often used as an example of a product of the flawed Tarkin doctrine. However, the TIEs are very much fighters with the lessons of the clone wars in mind. Firstly, the TIEs were only introduced shortly after the war concluded, and its first actions were fighting the CIS holdouts. Here the TIEs did well, and it is only after the next generation of starfighter designs becomes commonplace (X-wing, A-wing) do they suffer. The TIE was designed with logistics in mind. It has very few moving parts, is dead-simple to maintain, has no hyperdrive (as Venators would carry them, just like how it carried hyperdrive equipped fighters such as the ARC's and V-19s). The TIE goes all-in on anti-starfighter combat. It only carries 2 exceptional laser cannons, but it has superb speed and maneuverability to cut the CIS fighters to pieces. It was the result of the Republic needing to be able to clear the skies for its bombers and capital ships to decimate CIS formations. The TIE pairs well with the heavier Republic starfighters such as the ARC-170, so it's clear to see how well it would have performed within its original role.

The Imperial/Imperator class star destroyer is the posterchild for the Tarkin doctrine, yet was a product of the clone wars, not the doctrine itself. The ISD class was designed to supplement the Venators, as they were found to be undergunned and somewhat fragile in fleet battles. The ISD was never meant to be a carrier, nor replace the Venator wholesale, it was designed to fulfill a different role from the Venator. As the Imperator was never meant to be a carrier, a lot of its internal volume sits somewhat unused. I propose that the Imperator could carry more than its listed starfighter complement, but it uses its space to store more crew amenities, ground forces. It's only in the Imperial era that the ISD carries fewer fighters than it theoretically could, because it doesn't need to carry >100 starfighters when it is on patrol and fighting hilariously outmatched pirates. Also, since it was meant to supplement the Venator, if a taskforce needed its starfighters, it already had a Venator to do that. The Venator already has the long hangar deck and doors, trading protection for deckspace and the ISD does not.

Now, a big question revolves around the Venator. Why did it get replaced by the ISD wholesale, if the former was never meant to replace it? I think that the Venator was not actually replaced by the ISD, it is just that the older Venators in the fleet were decommissioned and scrapped, whereas the later production models were still kept in service. This has historical precedence as well, as many ships in the real world were decommissioned following the world wars, despite being relatively new. Following the end of the war, the Empire probably looked at the older and worn out Venators in their fleet and said: "We don't need them anymore, we have younger ones already in service, and the brand new Imperators/Imperials are coming online". Then nearly 20 years later the Rebellion shows up and the bulk of the Venators are gone, but not all of them. Some of them are still in service, but there are not nearly as many Venators as there are ISD's.

In the intervening years Tarkin makes his foolish doctrine, and he settles on using the biggest starship in service (outsider of the Mandators) to base his doctrine on - the ISD. This is why we see the ISD to serve in every role. Tarkin became obsessed with his own doctrine, using the ISD for too many roles instead of a diverse fleet. By the time the Rebellion really gets going, the Venator fleet is spread too thinly, and the ISD is forced to take up the Venator's role. On top of that, since the Empire hasn't had to fight seriously (small planetary wars and CIS holdouts aside), their ISD's have dedicated their internal volume to amenities and ground forces. Since this was done early on in the ISD's service life, it has become standard TOE for the ISD to carry a large land force instead of packing in more starfighters. As for picket/anti starfighter duty, the Empire does have the ships available and in production, but their doctrine doesn't see them deployed as often as they should be.

Eventually the doctrine gets out of hand and the Empire builds the Death Star and Executors, which is much to the dismay of the actual military commanders (Thrawn, Tagge, Vader, etc...). They know that these are somewhat unwieldy weapons, and indeed they were. Although, I would like to say that the Executors would be very useful in titanic fleet battles. The Executors are still "conventional" and at Jakku in canon and Orinda in Legends, they were very useful for those battles.