In this chapter, Gibbon starts out with his tabloid trash again. You do not get to be dux in a Roman province without training hard for 40 years and you don't get to be named chief of the elite cavalry by the emperor, in the environment of the empire at this time, without substantial patronage. The idea of the little slave boy that rose to be emperor is one of those popular urban legends about leaders, and it suits what Eutropius kept saying, Romans overcome their problems. So it's a theme, not a truth; the technical term is that Eutropius is tendentious, and writes to suit his tendency.
Gibbon sneers at Diocletian because he's not the stuff Gibbon would make a hero of.
With a plague devastating Rome to the tune of 2,000 people a day, Diocletian decided to make sure disorder did not get out of hand, and sent in Maximian who was nothing if he was not a soldier. But Maximian's career took a downturn about 290 CE and he fell afoul of Constantine's rising star, eventually committing suicide on the latter's prompting.
Diocletian lost Britain, a major source of silver and tin. The Roman lifestyle in Britain carried on for about another century in high density populations, but when the port of London was abandoned, traders hesitated to go the long way to the Cornish ports and imports slowed down. A hundred years later the Saxons re-developed London starting at Aldwich, and traded all around the eastern part of the island. Contrary to the preferred story of the later Victorians, their Germanic relatives did not take over a deserted island. The high percentage of Celtic DNA even in London argues against that, and in the 400s Irish priests restored learning to Europe. Of course, Gibbon would ignore this, sharing the English prejudice against the Irish, all the more so as this was a Catholic revival. St. Columba of Ireland worshipped according to the Welsh rite.
Finally, Diocletion split the empire. You can't say there was no going back after that. There was already no going back to anything, least of all to Gibbon's Roman Republic. Diocletian institutionalized abandonment of the west to the Germanic migrants, that's all. So you have to ask yourself, with the Vandals swanning around North Africa, what did the eastern empire do for grain?
Diocletian did two things about that. First, he increased irrigation, in the Levant, for one example. Ruling from Nicomedia, he had enough nearby grain production not to miss Egypt so much. Diocletian could do this because he confiscated uncultivated land from the rich and gave it to the government to operate. And he built reservoirs for bringing irrigation water to these lands.
https://scriptaclassica.org/index.php/sci/article/download/3399/2927
The second is, he created serfdom. That's basically what his coloni were. Serfs. After nationalizing land that the rich owned but did not cultivate, Diocletian not only needed to staff the agriculture, but also to stop all this nonsense about the peasantry rising from their class through patronage or whatever, and eventually being rich enough for a seat in the senate or on the throne.
After Diocletian, the marginal areas where he established irrigated fields were again abandoned. From 330 CE on, taxes paid in kind from Asia (Minor) and Bithynia would have gone to the eastern empire while Rome bought grain from the Vandals. And Diocletian's capital, Nicomedia, was already part of a large trade region, including fertile areas in the east, that had been paying taxes in kind for 200 years. Constantine's shift to Byzantium made transportation of grain from these regions easier because it came across the Black Sea, not across roads threatened by migrants.
https://ore.exeter.ac.uk/repository/bitstream/handle/10036/3713/GuneyH_TPC.pdf?sequence=3&isAllowed=y
It's not enough to say that Diocletian wanted at least part of the empire to not be invaded every few years. There would have been no Constantinople, except that Diocletian knew something about logistics, such as that his eastern domain could not survive without food. The pre-existing tax-in-kind setup was perfect to feed his empire.
What's more, this is why Gibbon is wrong about eunuchs. Everybody who talks about eunuchs says the same thing: they have no motive for grabbing gold because they have no heirs. Well, they have no genetic heirs, but can they make a will and testament? And if they can and do bequeath their gold to somebody, a) who better than the government and b) why wouldn't the government say no, you can't do that, it's ours, if they did bequeath it to somebody else. China was different; Cao Cao was not a eunuch, but he was adopted by a eunuch of the name of Cao, and that got him into the upper echelons of China's court.
But in the Roman world, a will could be set aside because it did not exist on the grounds of piety, and there is nobody who owes piety to a eunuch, nor could siblings get more than 1/4 of the amount in the will if they challenged it.
https://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Testamentum.html
So whatever the Roman government paid to eunuchs who were either officials or non-slave servants, it came back to the government, which can't be said of the possessions of people who can have children. Them you have to grind it out of with taxation, and that is exactly what Diocletian and those after him did.
And then again, there's that logistics thing. Eunuchs have no genetic families to feed. The grain coming into Nicomedia or Constantinople went 1) to the throne; 2) to the court, many of whom were eunuch officials or servants; 3) to paying customers like merchants, who earned the money by trading from the Black Sea across the Mediterranean, and who also employed eunuchs; 4) to the coloni who raised the grain and other food.
So basically, eunuchs are a benefit not a drag, and the government doesn't care how much gold its eunuchs pile up because all it has to do is make laws to confiscate it when they die.
And never underestimate greed. Though they have it but for their lifetimes, people who have no genetic heirs will not avoid accumulating wealth because of that. It's the cha-ching factor, the odometer rollover factor. Although at some point it becomes simply a number instead of the ability to do what you want, the lack of genetic heirs does not stop people from accumulating wealth. Modern examples include David Geffen, Giorgio Armani, the founder of Aldi markets who bought out the American Giant food stores, and so on. In a past age or in some parts of modern society, people may acquire wealth for their children, but it's not universal. It probably never has been.