This isn't true. Businessmen in Republican Rome were the social equivalents of trial lawyers today - wealthy buccaneers, sneered at by all the respectable folk.
The city even had mass production of some consumer items and a stock market.
Um, no. There is no surviving evidence of what we would recognize as a Roman "stock market".
Why did Rome decline and fall? The record is abundantly clear on this point.
The record is abundantly clear as to what happened, but nowhere near clear on why. One alternate theory is the Pirenne Thesis, which holds that, despite changes in governmental control, the Mediterranean world was basically okay until the coming of the Arabs in the eighth century, because they destroyed the maritime economy.
Early in the process, a politician named Clodius ran for the office of tribune on a "free wheat for the masses" platform and won.
Clodius? Eventually, yeah, but what about Saturninus or the Gracchi, a generation or two before?
The government responded by imposing penalties for trading in gold, especially for exporting it, much as Franklin Roosevelt did in 1933.
The imperial government banned the exportation of gold because the Roman Empire had a massive trade imbalance with India. Bullion was going east in exchange for luxury goods, and the Mediterranean world was being drained of specie.
Not only did he impose across-the-board wage and price controls in relative peacetime, but he also resigned from office, in the year 305. Nearly 17 centuries later, Richard Nixon would become the first American president to impose peacetime wage and price controls and also our first chief executive to resign from office.
Diocletian didn't "resign" like Nixon did; he retired in order to permit orderly succession under the Tetrarchy. This comparison is facile.
The once-proud Roman army, which had always repelled the barbarians before, now wilted in the face of opposition. Why risk life and limb to defend a corrupt and decaying society?
By 410 AD, the "once-proud Roman army" was itself the band of mercenaries which plundered Rome. National armed forces didn't exist in the sense which we understand them in the fifth century AD.