Gunpowder for ever changed the social relations of Europe over the course of six centuries by gradually rendering irrelevant the traditional reciprocal obligation between knights and peasants, and the natural moral basis for the rule of the knightly aristocracy; with the First World War came the death knell of the warrior aristocracy. We cannot intelligibly return to the particular arrangements of the old system, but on the other hand no adæquate replacement has yet been found.
The process began earlier in China, during the early Song dynasty, when the replacement of the traditional aristocracy with an exam-selected bureaucracy happened to align with the loss of Tang dynasty horse-rearing lands and the replacement of cavalry tactics with massed crossbows and eventually gunpowder arms. The reduction of the warrior aristocracy’s political power was intended to avoid repeating the fall of the Tang dynasty by denying generals the use of local power bases, but it reoriented even the education system. The massive use of an examination system to populate the imperial civil service, replacing the traditional aristocratic recommendation system, oriented Confucian classical education toward success in examinations. For the last thousand years, therefore, to meet the needs of the imperial bureaucracy, Chinese classical education has increasingly stressed memorization and model answers over the dialogue seen in the Analects and the Mencius, despite some countervailing tendencies in the Qing dynasty’s (1644–1912) bannerman military aristocracy. In a process parallel to that experienced by Europe, then, one may see in China a sea change in social structure over the course of the last thousand years.
What kind of system is to replace, by translation for new material conditions, the system that upheld traditional ideals and cultivated virtue in the age before gunpowder?
Cogito, Credo, Petam 
A good reason for the decline for the Military Aristocracy is to ensure greater Military competence. But if Military competence is reduced as a result over the long term.
What use then is the loss of Military Aristocracy?
In regards to China. You suspect the loss of such Military Aristocracy is ultimately responsible for their Military decline?
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I actually suspect that the loss of a robust military aristocracy is at least partly responsible for an ethical decline, including most likely a moral decline.
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Now if only such a Military Aristocracy can be restored. Not exactly the same of course. But perhaps conditions can take place to make that happen.
When the Western Roman Empire collapsed. Germanic peoples set up their respective Military Aristocracies over time.
The development of Heavy Cavalry by the Franks evolved in the Knightly Class of Europe.
Studying how they arose in the first place may give us clues.
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But then I suspect the decline of the Military Aristocracy causes another problem.
“The society that separates its scholars from its warriors, will have its thinking done by cowards and its fighting done by fools.”
-Thucydides
The Intellectuals of old like Socrates was himself a Hoplite. And Plato was a Strongman.
How many Chinese literati were physically fit and grounded in their physicality in dealing with the world rather than being lost in abstraction?
Christianity involves the Incarnation of God in a Physical Body. The joining of Heaven and Earth.
The Physical isn’t separate from the Spiritual:
“The Hebraic view of man as an animated body and its refusal to make any clear-cut division into soul and body militates against the making of so radical a distinction between material and spiritual, ceremonial and ethical effects.”
https://www.tektonics.org/af/baptismneed.php
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