The Orthosphere ponders if Traditional Christian sects can coexist peacefully. The author basis his question on the idea that "eventually" Islam and modernism will run out of steam and Traditional Christian churches will no longer have the common interests that currently force them into cooperation and coexistence.
There are points at which one may fault the author.
One can argue that modernism hasn't been an issue since mid-twentieth century and we are in the grip of post-modernity, a nearly apocalyptic (in the Mad Max sense of the term) intellectual wasteland in which even modernism's tepid context based appreciation of religion, culture, and tradition have been displaced by an unrelenting impulse to level everything irrelevant arbitrary constructs. In this respect, one encounters the ultimate weakness of the Traditionalist movement; as whole, many of its adherence are fighting an intellectual war that they not only lost many years ago, but has since been displaced by much larger conflagration.
Islam is another matter. Assuming that there will be any decline in Islam defies common sense. Islam is growing and shows no signs of abating. Its growth is not only through birthrates, but in its appeal among members of Western societies who are looking for a stable spiritual tradition. Granted, the appeal in the West is often towards more mystical Islamic sects, but growth is growth and it comes by way of the self inflicted wounds on the Western tradition.
The author also fails to recognize the major impetus behind the move towards ecumenism. The experience of the first World War raised serious questions Christianity had to reckon with. Among them, how is it acceptable for people who ostensibly worship the same God and hold many of the same core traditions to participate in a conflict that invariably involves killing one another? Granted, this had occurred many times in Christian history. The early 20th century, however, benefited from the influence of modernity in that there was an intellectual environment that encouraged contextualization and the conviction that one is not necessarily bound to the conventions of a previous era. Modernism, like most philosophical systems before it, at least offered one item for assimilation by Christianity - the appreciation of historical context and the reasonable limits of said context on the present.
Any argument that ecumenism exists solely on account of the existence of "enemies" is somewhat short sighted. The more poignant question is what will be the conclusion of ecumenism, unity or coexistence?
Parties that push for unity are often ingenious to the "problem of unity." Unity, invariably, has winners and losers. So far as ecclesiology goes, unity requires someone (Rome, Orthodoxy, or Protestantism) renounce its ecclesiology in favor of another's. This is a conclusion that is impossible to avoid. Ultimately, someone is going to give up their ecclesiology and the stakes are high no matter how you approach it. Even if the ultimate ecclesiology born by unity is factually correct, it requires major religious re-engineering on at least two sides (one side, overall, would have minimum adjustments to make).
Coexistence, meanwhile, leaves ecclesiology, and hence doctrinal and structural reform, untouched. True, coexistence almost certainly eliminates the possibility of inter communion. Yet, it also acknowledges the historical development of Christianity and upholds the corresponding distinctions among the notable Christian confessions, avoiding the consequence of homogenization. In some respects, coexistence is the most honest option. It avoids applying anyone's "one true church" theory across the ecclesiastical spectrum. Coexistence may also be thought to be more "moral" in so far as it avoids the probable disruption caused by the religious re-engineering needed to acclimate to ecclesiological change.
Is it possible that coexistence itself could fail? This would largely be a societal matter. Where a given confession has a dominant social influence or official status, it is probable that coexistence will fail - at least this is the example we have from Christian history. However, such official designations are relatively rare among societies whose inhabitants are in the majority Christian, contemporary Russia perhaps being an exception.
Barring a societal collapse and the emergence of conditions in which only institutional religion can sustain rudimentary social, political, and economic needs, it seems unlikely conditions unfavorable to coexistence will take hold in the west. As such, the basic impetus to know one's neighbor exercises definitive influence among various Christian confessions with hope of maintaining healthy social relationships.
In other words, only the most pathological of antisocial tendencies would make one think one can contemplate the end of coexistence as a possibility.