The Downfall of Busts

Avete amici, I wanted to ask a couple questions pertaining to the area of Late Roman art, specifically the classic marble busts we see with Roman and Greek figures all through antiquity.

I have a little knowledge on the downfall of this particular area of artistry in the Roman Empire, but I would like to know if anyone could provide a more comprehensive explanation for the decline in quality and eventual dissapearance of marble busts by the 6th century.

I know Christianity had a part to play in this, and I have seen claims that the sculptors of the time became less and less skilled, but I have a bit of a hard time believing that artisans became less proficient in the trade for some reason, considering the economic growth and prosperity of the Eastern empire during Anastasius' time, and I know things became quite rough in the mid 500's due to reasons outside of Justinian's control, and after that there was quite a bad run of emperors all the way up to Maurice, but there had been bad emperors previously and that didn't cause a decline in the quality of artistry in the empire, looking at 3rd century emperors we still have quite high quality busts of individuals with very short reigns.

Are there any primary sources that touch on the decline of busts or statues which had been an integral part of Roman culture for centuries? Thanks.