Yep i do hear this argument all the time. But the question is why the integration did not happen?
what are the factors which make sure that integration takes place? A minority living in an area is more likely to integrate with the majority, that is a universal truth. In Sindh, the urdu speaking were living mainly in URBAN cities and they were in HUGE MAJORITY, Karachi and Hyderabad are the examples. In interior Sindh there were very few MUHAJIRS, and they DID integrate with the Sindhi culture. But the lines were drawn NOT in INTERIOR SINDH, it was URBAN SINDH.
In Punjab we didn't have this problem, the population distribution was in right proportion in Punjab. Sindhis maily rellied upon irrigation so they didn't move to cities, where as Muhajirs replaced Hindu middle class in main cities. With the time when Sindhis started moving toward cities they realised that they are like strangers in their own SINDHI CITIES, you can't blame them for this, nor the blame can be put on MUHAJIRS since the future prospects for them were in cities, not in small villages.
example from recent history, there are about 3 million POLISH nationals migrated to UK in recent years, and they are blended well with the British society even though they have different culture and language. The key is that they are not limited or headed to only ONE AREA, they are all over UK.