Those smart cities or tier II, III cities won't be as cosmopolitan as Mumbai, Delhi, Chennai, Bangalore or other big cities currently existing... The mindset of the people is the main issue here...
Im saying with time they will be. My home city Coimbatore (tier 2) already has plenty of Bengalis and others coming in and setting up the groundwork with all kinds of businesses and such or working in IT parks, managerial or engineering roles in the larger facilities like Bosch etc....or studying in TNAU and other universities and colleges.
There are like 20 - 30 existing tier 2 cities of this size....and many dozens more will become similar tier 2+ cities.
Also, since, we are talking about West Bengal here, it's a state of about 100 million people, that's pretty large...
Not all of W.B is going to face such problems (even hypothetically) just like all of Bangladesh wont either....and definitely not in a very short time frame of say under 20 years either. Well within absorption and ramping up capacity of Tier I and Tier 2 cities including the ones that join these brackets over time much more quickly than even today.
Bengalis will always be welcome in Tamil Nadu I can say personally. We much admire their culture and contribution to India. Our elders looked upon Bengalis with quite some awe and wonder and taught us that this is another worthy distinct culture of India just like Tamil culture....and there is much scope of synthesis and mutual admiration, cooperation and integration: