The one issue with this scenario is that it presumes the Serbs, Bulgars, Venetians, and everyone else would focus most of their energy on subduing the Byzantines, instead of fighting each other, or having a situation where one of them gets too powerful, resulting in shifting of alliances. While getting Byzantium to pre-1204 levels would be next to impossible without something akin to divine intervention, it does not seem too far-fetched if the Byzantines regain OTL modern-day Greek territory with few modifications to account for them only holding coastal areas, plus coastal parts of Smyrna - thus they are strong enough to fend for themselves and not to fall to any of the rival powers, but not strong enough to cause mischief and to shift the overall balance.
And while the Byzantine trading income diminished during the XIVth century, there are indeed attestations that even during the reign of Andronicus III, the Empire was still capable of both raising significant manpower (Andronicus' army as he marched against his grandfather is said to have numbered aroud 50,000 or so men), and still in possession of somewhat significant wealth (again, Andronicus was described as being "Europe's richest suzerain" in terms of having the higher income than most other kings). Thus, as late as 1340s the resources for a potential rebound seem to still have been there.
Now, a word of caution on the previous paragraph. My source on that is Edward Gibbon, and I am not sure how much of an authority he should be considered on all things Byzantine. I recall reading a number of other sources having to do with economic life in XIVth century Byzantium, but unfortunately do not remember the exact references off the top of my head - all, however, describe a society that, from economic standpoint, was on par with most European ones of the time.