Looking at Roman coins, beside their ever decreasing silver content, there's undeniable decrease in artistic appearance in the last 500 years of the Empire. To say nothing about Byzant coins that came later. Do you have an explanation to this?

A big reason would overall collapse (More advanced reasons could be found by looking at the decline of the Roman empire in general after 200AD).
Christianity is one reason. Another is the 3rd century decline after the endless run of bad and short term emperors after the death of Septimius Severus in 211AD. There were some good ones (Constantine, Aurelian, Majoran etc) right up to the end, but many more bad ones like Elagabulus, all 3 Gordians, Honorious, Maximus Thrax etc.
I studied Roman Art papers at university and you notice a decline in sculpture from the late 2nd century onwards and by the 3rd century you are getting heavily stylised art in the place of accurate and realistic art of the Republican era to Severus. Realistic looking art persists into the 3rd century as your coin of Clodius (260s) shows, but by 300 and the Christian wave of stupidity descending upon it heads towards toyland levels and does not become realistic again until Giotto and Cimabue around 1275 AD.
Plus from the 3rd century (200s AD) you are getting economic chaos, with coins becoming more worthless and debased over time. In 100AD the silver denarius and brass sesterces were the king of coins, by 300 AD its a worthless billon antoniannus and copper follis/radiate/s
I also suspect the guild of coin making got easier to infliltrate and by 300AD you had roman coins being minted throughout the empire, with virtually anyone making coins, more barbarians and non Roman citizens involved in the process, economic collapse (Which meant skimping on metals, design and execution - flooding the market with low quality billon and copper coins). Add to that a lot of fakes and by 400AD in many places, coins had disappeared and been melted down into weapons, buried or melted down and mixed with copper to make cheaper and junkier coins. Ever notice most “Roman coins $1 each” are usually worthless lumps of copper, that if you can read anything will be one of the Constantines or some later 4th century emperor. Well made and deatiled round coins, gave way to shapeless lumps of copper with childish faces and basic runic style writing by 350AD.
On top of all this, add in the dumbing down effect of Christianity, which killed off all creativity and the engineering flair and genius that made Rome great. I could go on, but won't.
I am not sure if it is just about declining skills. It might just be that the preferred style of art changed. Photorealism is not the pinnacle of artistic expression after all. There were horrible periods of crisis when the classical realistic style continued without any problems. The change in style could well have been intentional.
Moneytane
A big reason would overall collapse (More advanced reasons could be found by looking at the decline of the Roman empire in general after 200AD).
Christianity is one reason. Another is the 3rd century decline after the endless run of bad and short term emperors after the death of Septimius Severus in 211AD. There were some good ones (Constantine, Aurelian, Majoran etc) right up to the end, but many more bad ones like Elagabulus, all 3 Gordians, Honorious, Maximus Thrax etc.
I studied Roman Art papers at university and you notice a decline in sculpture from the late 2nd century onwards and by the 3rd century you are getting heavily stylised art in the place of accurate and realistic art of the Republican era to Severus. Realistic looking art persists into the 3rd century as your coin of Clodius (260s) shows, but by 300 and the Christian wave of stupidity descending upon it heads towards toyland levels and does not become realistic again until Giotto and Cimabue around 1275 AD.
Plus from the 3rd century (200s AD) you are getting economic chaos, with coins becoming more worthless and debased over time. In 100AD the silver denarius and brass sesterces were the king of coins, by 300 AD its a worthless billon antoniannus and copper follis/radiate/s
I also suspect the guild of coin making got easier to infliltrate and by 300AD you had roman coins being minted throughout the empire, with virtually anyone making coins, more barbarians and non Roman citizens involved in the process, economic collapse (Which meant skimping on metals, design and execution - flooding the market with low quality billon and copper coins). Add to that a lot of fakes and by 400AD in many places, coins had disappeared and been melted down into weapons, buried or melted down and mixed with copper to make cheaper and junkier coins. Ever notice most “Roman coins $1 each” are usually worthless lumps of copper, that if you can read anything will be one of the Constantines or some later 4th century emperor. Well made and deatiled round coins, gave way to shapeless lumps of copper with childish faces and basic runic style writing by 350AD.
On top of all this, add in the dumbing down effect of Christianity, which killed off all creativity and the engineering flair and genius that made Rome great. I could go on, but won't.
Excellent observation and analysis!
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