Could not be said better.... Bangladeshies are a mongrel nation of many bloodlines particularly the muslims. Only thing that binds is Islam, language and a broad perspective.
However, Mongrel is a derogatory term for any group of people. We can better attribute the term "A mixed group" of people in Muslim Bengal. But, it is certainly not unique for Bengal. This phenomenon has gone through centuries among all the groups of people on this Earth in the pre-historic as well as historical times.
For example, in the USA most of the whites have been mixed with each other after the immigration a few centuries ago. Blacks were not basically a homogeneous group of people as well. Rather, they were brought from many different parts of Africa. They became mixed among themselves as well as with whites during the course of time.
Many similar things happened in the past throughout the world. People moved out to another place and mixed with the local people during the course of time. So, we do not have to derogate us. By reading the political history (1203 to 1757) of Bengal a person will know that except
Raja Ganesh (a Brahmin) all the Maliks, Sultans, Subedars and Nawabs including Alivardy Khan/Sirajuddoulah were of foreign descent. His son, Jitmal (Jodu), had to accept Islam to continue his father's line.
It was certainly not that the outsiders came alone and declared themselves the Sovereigns of Bengal. Many thousand others accompanied them to seek a better living. They have mixed with the local converted Muslims gradually.
People now do not really know who is what, and whose forefathers were what. They live in the present homogeneous society bound by one religion and one language.