Problem is that Pirenne's thesis, altough being historically one of the works that led to understanding the period between the IIIrd and the Xth centuries being a long transition between "classical" Antiquity and "classical" Middle-Ages, doesn't fit the historical and archeological sum of knowledge we have on the period.

These answers by u/bitparty and u/Tiako should give you a good insight, and among other points,

- Mediterranean trade regression was already perceptible in southern Gaul already by the early VIIth century : reasons for aren't well determined, altough it might have something to do with the growing impopularity of the Empire (for religious reasons, in particular), possibly the consequences of Justinian Plague's (that seemingly hit harder the Eastern Med. basin than the western, and less so Northern Europe), and its unability to fuel a diplomatic trade in the western kingdoms due to the cost of the wars in Central Mediterranean basin and against Persians.

- Western societies seems to have felt little interest in the Arabo-Islamic conquest of the VIIth century while being aware of them. Both pontifical and Frankish account them only when they are at their doors, without accusing them to have "broke" the Mediterranean trade trough conquest or piracy (something that really began to appear later in the IXth century).

- Links between Constantinople and the former western provinces of the Empire were quite stretched thin by the VIIIth century : kings gave up the fiction of being subservient to the emperor, for instance by replacing them on their coinage with their own names and regalian depiction (as soon as the mid-VIth century in Francia). Parchemin already replaced papyrus, bee wax replaced olive oil, and critically gold coinage was replaced by silver coinage in the late VIIth century, illustrating an economic rupture.

Chris Wickham was already mentioned on the "why" of the relatively gradual economic degradation in western Europe : the conscious de-growth having possible parallels in post-Imperial Britain^(1), altough there's room to understand it not only as a social-economic empowerment for the lower class but as much an understood limitation from local elites entering into a transformation giving them political agency as well : that said the absence of an overbearing authority enforcing taxation, service, rent and clientelism over its peasantry and its replacement by weaker, more regional authorities might have reinforced the former's agency both in familial groups, but as well communal either in autonomy or in relation with a land-owning and/or political patron (aristocratic or clerical). At least in Merovingian Gaul, slavery might have diminished (or be more "humane") and lower classes in a relatively fair health compared to their descendents (up to the XIIIth century, maybe up to the XIXth in some aspects).

Richard Hodges, in "Dark Age Economics" (and its follow-up "A New Audit", published after Wickham's "Framing the Middle-Ages" and taking it in consideration), proposes a perception of this early medieval trade related to states and institutions as relayers and motivators of exchange.

Even in the Vth century, the Roman trade network took several bullets due to the collapse of the Roman state and what ensured the existence of a monetarized economy. It'd be felt more heavily at the periphery, outside the Empire itself in Germania, causing migrations and land abandonment, notably in Frisia. In western Romania itself, again more heavily felt at its extremities, while somptuatory and utilitarian trade was kept alive, economy underwent a simplification both in exchanges themselves and how they happened.

As the "dust settled", with new polities forming out of Vth century warlordism in western Romania, it let demesnes and peasant communities with a certain autonomy (far less welcomed among Roman aristocracy than it was probably the case among peasants, Sidonius Apollinaris complaing to Riothamus that Brittons took slaves from their owners, for instance) relatively to a closer but weaker palatial court. While the need of a strong surproduction was gone, the need of prestigious goods to display one social status or romanity wasn't and with the disappearance of an multi-regional economy, the new political make-up might have been determinating, new kings presiding over the regional exchanges : goods sent from the Mediterranean basin, overwhelmingly by the Roman Empire, found their ways in western Europe (which is especially true for prestige goods) by trade roads mirroring the new courts and centers as well as relations of clientelism (Ostrogoths to Bavarians, Franks to Kent, etc.) carrying over goods after the original traders left them, having took utilitarian goods in exchange (grain, leather, meatals, slaves, etc.). It's not to say that courts monopolized the presidence of exchange (if anything, Merovingian villae seems to have acted as some sort of "little courts" for local rural exchanges, either under or in parallel to main roads), but that they probably have been privilegied actors in what mirror trade in pre-Roman Europe.

Top-down redistribution (rather than trade) certainly played an important role into the geographic homogenisation of prestige goods, but probably not the only one especially as trends of politic regionalisation went in full force by the VIth century (arguably later in Francia). Merchants, in western Europe, were likely not independent traders but agents tied to an household or service, altough with their own agency : people more akin to managers, purveyors, diplomats, translators or even public-servents making a profit out of the trade or their service. Necessary intermediaries over "exclusive" trade roads (understanding there that, for instance, Frankish or Kentish agents controlled exchanges or tolls over their jurisdiction), these definitions could very well apply to travelling monks or bishops, serving in courts or along aristocratic families they were very often part of when not at the service of their own religious demesnes (such as Noiremoutier's monastery). Eventually, and this particularily applies to Mediterranean trade, there were individuals from minorities ("Syrians" up to the VIIth, Radhanite Jews during the Carolingian period) that ensured, partly out of a "pearled" presence along the trade ways (Isaac of Narbonne being sent as an emissary from Charlemagne to Harun al-Rachid).

This "institutionality" of trade both led to economic growth and to expand to new partners as new polities emerged further down the line (with their own partners); but also a renewed control for the actors of the trade, such as the aristocracy, the state or the church (which weren't as hegemonic or mutually exclusive, if competing) in productive organisation (triennalization of soils, organization of surplus, etc.) but as well coinage politics, trade markets planning, etc.

The development of a North Sea trade would be then less contradictory and more dependent for its existence on a maintained Mediterranean trade in the wake of Rome's fall. While the old Mediterranean network was exhausted, it wasn't entierelty dead and found some other ways to reach North Sea trade with Arabs (either trough Franks that dominated the connection between them, either by-passing them) but the collapse of Carolingian Francia (which, in some aspects, had several hardships dealing with agricultural crisis and effects of reestablishing a Frankish hegemony beyond the Rhine in a "too big for your own good" way) did a number on macro-regional trade, with links with the Meditteranean basin being relinked in Italy and the Byzantine "renaissance" of the Xth/XIth centuries.

^(1) The Ruin of Roman Britain - An Archeological Perspective; James Gerrards; 2013; Cambridge University Press)