If it's not plausible, what's the point though? You could also posit a scenario in which Rome conquered Germania and Scythia and Pahlava under Trajan and ushered in a golden age of Roman rule which lasted for 2 millennia. Sure it's a thing, but from a historical perspective the thing that makes alt-history so interesting is that it investigates the importance of individual events or characters on the flow of history. Not only are you not really saying anything of any interest or impact (other than that you have a hard on for the Roman Empire), but at this point you're just telling an outright fantasy, rather than performing a historical exercise through the medium of fiction.
To take Dachs' Eurasian War: it's interesting (as Dachs noted previously) because it highlights the tenuousness of the Entente in the leadup to the Great War, while also showing the institutional weakness of the British Navy, the potential for the collapse of both Great Britain and Austria-Hungary, as well as a bunch of other things. It counters the notion that World War I (as we know it) was "inevitable", while simultaneously challenging other common misconceptions about the period. This scenario is interesting and enlightening because the point of departure - A Russian victory over Japan leading to a dramatic realignment of the alliances in Europe, is both plausible and possible. If you don't build a good base to start on you're just pissing in the wind.
Swinging back to this historical fiction you gave us: what does this novel tell us about the Black Death, or disease in the medieval period in general? Is it supposed to tell us that Europe was lucky that the bubonic plague wasn't absurdly fatal? Or that diseases can be debilitating in a world without proper (modern) medical facilities and drugs? If you replace the Plague here with a zombie apocalypse or the advent of the rapture in 1348 would the story be fundamentally different? While it may be interesting from a philosophical standpoint, I don't think the story sounds at all interesting from a historical one.
It's not about the plague itself, it's about what happens afterward as Muslims repopulate Europe and how non-European societies were never conquered as easily. It's just one entirely fabricated, alien event, and then the rest proceeds rationally.