Yes Pak is multi ethnic, but all ethnicities have lived on banks of River Indus since centuries, just like Nile in Egypt. Pakistan the word is an acronym Punjab, Afghania (kpk old name), Kashmir , Sindh, balochisTAN ..1st time used in 1933 by Ch Rehmat Ali , Bengal was never in it. Pakistan resolution 1940 called for more than one state of muslims , but British never gave the option. Thus the state with two wings 2000 km apart was formed, separated by hostile territory. If we go this much distance in the west, we would reach Iraq & jordan, can pak & Iraq be one country, hell no !!!
Bengalis are absolutely different, with an ancient language & rich culture, with even their own calender. Indian Punjabis, Afghan Pushtuns, Iranian Baloch are simmilar to people of Pakistan....Bengalis are as different as arabs or berbers are for Pakistanis !!!