Obviously the term
multipurpose dam has escaped your notice, being an ADHD halfwit!
Utter falsehood. Erstwhile king Bir Bikram Manikya clearly stated his wish to join the Indian Union prior to his death in '47 according to the wish of his subjects, which was duly fulfilled by the Queen in 1949 by signing the Merger Agreement.
Your understanding is wrong, large public sector industries are there in Assam to make use of our resources. And we export lots of natural resources too to earn revenue. Private sector investment was low in NE due to militancy supported by China and your Bangladesh, and due to the ILP system. With the end of insurgency now in sight, investment in that sector is picking up aided by BJP's Look East Policy. NE will be the gateway to South East Asia in the coming years via Myanmar and infrastructure is being built on a war footing since 2014.
https://industries.assam.gov.in/portlet-innerpage/large-industries
http://www.neportal.org/northeastfiles/Assam/Industry/Units/Major_Industries.asp
A region blessed with natural resources can make up easily despite a late start, but the unfortunate ones like Bangladesh will be in trouble if they fail to diversify with their meagre resource base.