I can't think of a single example of a heterogeneous society that has lasted a comparably long time. Would you care to point to one, or cite some studies to show their effectiveness vs those that are homogeneous in nature?
I'd argue that a strong sense of shared identity, such as that provided previously by religion in the West, and which still currently holds strength elsewhere, is crucial to the success of a given society. The War state, and then post-War state, of the West also had a strong sense of shared identity and bound together as a result. Evidence points towards it.
The current state of the West is a testament to the disarray caused in a heterogeneous society with no sense of shared identity and the mutual obligations that result from it. The divisions and gaps are only widening, the issues around it are only becoming worse, and those who stand to benefit from it are using it to its full advantage.