I don't know enough about sculpture, but regarding architecture, there were many innovations during Middle Ages.

First of all, medieval builders constructed churches and palaces that were bigger and higher than ancient ones. That's what Gothic architecture is all about (even though the name "Gothic" is a later pejorative). This was possible because of new innovations in structure of arches etc.

They were also able to construct these buildings with real windows, which was not actually the case in ancient times. (For instance, look at Pantheon: ancient, and quite big and impressive, but no windows, just the oculus at top).

For construction tools, medieval times came up with e.g. the wheelbarrow (although this was independently developed in China much earlier) and various types of cranes.

If we look at politics, law and economics, well, medieval politics was sophisticated in many ways and I cannot see why we should call it regression. As I wrote in another comment, during Middle Ages the development achieved, for instance, abolition of slavery and things like Magna Carta. As well as the start of merchant banking and foreign exchange contracts.

And many of our well-known poetic legends are medieval: Roland's Song, Carmina Burana, The Canterbury Tales, the Divine Comedy, Beowulf, and if you go to religious arts, people like St. Francis.

Edit: regarding sculpture, I'd imagine that the Greek had easy access to sculptable stone materials that medieval Central Europeans had less so. There's even less stone sculpture in northern Europe where the bedrock is granite and it takes absolutely modern tools to sculpt them.

Not true for pretty much all of that. A great deal of the advances made by the Romans were simply unworkable in Northern Europe. Roman concrete, for example, depended on volcanic pozzolanic ash which was not available further north. Roman agriculture was based on simple scratch ploughs that took advantage of the sandy soils around the Mediterranean. To work the hard, dense soils of the north required the invention of the heavy carruca, leading to a dramatic increase in food production and a population explosion through the middle ages.

You can keep on going down the list. Medieval poetry and literature developed extensively into forms we take for granted within our modern culture. The ideas of chivalry, nobility, and courtly love are so deeply ingrained within our society that we take them for granted.

As for architecture? Come on! Take a look at Aachen Cathedral[0] (built in 800 AD) and tell me that's a "regression" compared to the Romans.

[0] https://upload.wikimedia.org/wikipedia/commons/6/69/Aachen_C...