Thrawn was analyzing the flaws of the Empire's centralized system.
He was concerned about the risk of concentrating too much power in one person, and the huge costs and exhaustion associated with maintaining the system.
As long as the Emperor is alive and governing alone, the structure is safe, but as long as the Emperor is human, there is no telling when an unforeseen event will occur. It is dangerous in terms of security. If the Emperor dies, the Imperial system will collapse all at once.
The enormous costs of running and maintaining a galaxy-scale dictatorship are huge, and Thrawn saw the exhaustion within the Empire. The Empire was responsible not only for military strategy, but for the entire galaxy, as well as for national administration.
The Emperor was aware of Thrawn's warning. However, the Empire was only as good as its own power, and lifting the centralized system would be tantamount to the collapse of the system. That is why Thrawn was opposed. They are the yes-men who surround the Emperor.
Thrawn's proposals of "low-cost galactic governance based on efficiency" and "non-state actors specializing in military strategy" are advanced and groundbreaking. They are completely different from Tarkin's doctrine during the Empire's heyday.
After the death of the Emperor, Thrawn, who refused to become the second Emperor, put his plan into action. He reorganized the Empire, whose system had collapsed.
He distributed the authority that the Emperor had to multiple staff members, creating a flexible structure that fundamentally denies the concentration of power. The era of relying on a single person is over. It overcomes security concerns and is efficient at the same time. By denying a centralized system and being passive about running the country, the cost of maintaining the system was greatly reduced. It avoids troublesome factional disputes and power struggles, and allows the pursuit of pure military strategy. (The New Republic is responsible for running the country.)
The Sith Eternal kept a close eye on Thrawn and the remnants of the Empire.
It is certainly admirable that Thrawn has not only overcome the flaws of the Empire, but also created the next generation of the Empire. It is wonderful. The Sith Eternals, who had been struggling to revive the Emperor, found it convenient to have a system that allowed them to flexibly govern the galaxy even without a dictator. How would the Emperor have evaluated it if he were alive?
And so they devised a plan to take over the plan.
Thrawn's plan was certainly advanced and excellent, but if there was no dictator forever, it would eventually develop into a power struggle and collapse. No matter how excellent the system, it is human nature to be jealous of the seat of power. Thrawn was not human, so he was unable to be wary of it.
Even if the system is perfect, humans are not perfect.
So the Sith Eternals used the status of Supreme Leader Snoke. He has a deliberate vulnerability, is by no means a second Emperor, and does not greedily seek power like the Emperor and does not centralize it. In other words, he is more of a system administrator than a dictator.
Although he manages a decentralized empire and has decision-making power, he lets his staff members take the initiative and builds a flexible system. He can flexibly deal with any unforeseen circumstances that may arise for Snoke. The costs of maintaining the system were greatly reduced. Therefore, structurally, the burden was not placed on Snoke alone.
Those around him were not yes-men, reducing the risk of them making the wrong decisions. They were able to come up with purely superior military strategies. They were able to achieve maximum results with minimal action.
The Sith Eternal had completed Thrawn's plan. It was the First Order plan.