This article arguing that all stolen art is on a level seems to make the error of assuming that the only consideration is whether a crime was originally committed, and not also (for example) the moral obligations of current possessors, the length of possession, the conditions of its being seized, the general curatorial responsibility for maintaining art, and the like, and also not to make an adequately sharp distinction between what is obligatory and what is merely good. On small scales and for individual pieces these don't always make much of a difference, but on large scales the moral interactions are rather more complicated. (Although it does seem a very good idea for museums to explain how each item in their collection came into their possession.)
Consider just one kind of complication. The bulk of the major churches of northern Europe, all of which are themselves highly expensive works of art, and the subsidiary art belonging to them, as well as monasteries, were seized from the Catholic Church by Protestant governments. Is this a theft or is it a legal shifting of jurisdiction (albeit under a legal regime that would certainly be regarded as absurd today)? If the latter, how does it differ from the appropriation of the Elgin marbles by the Ottoman Empire -- which had unquestioned legal control at the time? And whose shared heritage matters? Salisbury Cathedral and its clock are part of the shared heritage both of all the English people (most of whom are no longer Catholic) and of all Catholics throughout the world. I raise these points not because I think it's reasonable to demand that the Church of England give back all the seized churches -- this would be an unreasonable and pointless demand requiring the modern C of E, consisting entirely of people who did not actually cause the problem, to do something massively difficult from which they would not benefit at all -- but because whose heritage, whose tradition, whose jurisdiction, whose culture is in play is not a straightforward question.
But the single most important take-away is that it is possible to grow up to be a Professor of Art Crime, which admittedly makes for a more striking CV in every way. (A small field, though.)