I think the big problem with Pirenne is his use of the literary evidence and the way he thought trade worked. Pirenne approached the problem mostly through literary sources, but late-antique writers were generally not interested in trade and tend to only mention traders in a rather off-hand way. Using these occasional references, Pirenne argued that long-distance trade was alive and well until the early seventh century, but it is very easy to see how a few mentions of Syrian merchants in Marseilles from the History of Gregory of Tours for example doesn't really tell us that much about how trade actually worked. This type of evidence also only really works for luxury goods, since cheap pottery, textiles and even grain were hardly mentioned by our sources, who were generally part of the elite of society and not interested in such banal facts. Basically, a study of only the literary sources cannot hope to reconstruct the Roman/post-Roman economy.

Nowadays we have a lot of archaeological evidence to help us reconstruct this, to the extent that it is quite clear that long-distance trade was highly dependent on state intervention, rather than this pseudo-free trade framework as suggested by Pirenne. When the government was strong, the grain fleets from North Africa/Egypt to Rome/Constantinople also allowed other goods to piggyback on these lucrative trade routes, promoting economic growth throughout the Mediterranean. The state was also a huge consumer, since the military obviously needed a huge amount of supply and even the taxation system transferred wealth from one place to another, generating revenue along the way. However, with the fall of North Africa to the Vandals and the increasing inability of the Western Roman Empire to maintain stability in the provinces, long-distance trade began to break down in the West. The East in the fifth century on the other hand was doing rather well, but long-distance trade with the West decreased significantly. The destruction wrought by the Gothic War and the Justinianic Plague in the sixth century made things worse, so by the end of the century luxury goods can only really be found in port cities that retained a powerful aristocracy, as only they can afford the cost of buying luxury goods from across the sea. With this in mind, the Arab Conquests definitely did not transform the Western European economy, since the complex economic networks of the high empire had already been decaying for two centuries. I still agree with his classic line that "Charlemagne, without Muhammad, would be inconceivable", but I think we have to look at the political and cultural connections linking together the late-antique world rather than the economic ones.

For further reading, I highly recommend Chris Wickham's Framing the Middle Ages: Europe and the Mediterranean, 400-800 (2005) and Michael McCormick's Origins of the European Economy: Communications and Commerce A.D. 300-900 (2002), as both historians sought to replace Pirenne's model systematically. The Pirenne Thesis has long been attacked, but before these two huge tomes were published historians still have to engage with Pirenne's ideas. Nowadays the focus is on Wickham and McCormick. I especially recommend Wickham's book as he gathered together archaeological evidence from pretty much every region of Europe (which is unprecedented) and used comparative analysis to draw out a general pattern that is I think very convincing.

So a lot of Pirenne's arguments have come under fire for his characterization of the stagnation of the economy in the Frankish world, he uses the absence of papyrus as an example of the lack of long distance communication. To Pirenne he was big on long distance trade and gold standard coinage. However recently a lot of this has been challenged, with A. Verhulst arguing that instead of long distance trade the primary motor of the economy was instead the bipartite estate system. Michael McCormick's book also paints a drastically different picture of how the economy works, but in the end he argued that the Muslim conquests were still pivotal, but for different reasons. Verhulst's book is Rural and Urban Aspects of Early Medieval Northwest Europe and McCormick's book is Origins of the European Economy: Communications and Commerce A.D. 300-900 (Its about 800 pages long so I would recommend reading his article first, titled "A New Light on the Dark Ages" which sums up a lot of his conclusions)

Hopefully this helps

So from what I call tell .trade had already been on the decline .There was still actual some long distance communication in the Carolingian empire So most medievalists think he is wrong but for different reasons. Do you know any medievalists who agree with his views. Also any good books on medieval slavery.