Instead of
going into detail lets make it to the point. From where you have got the imaginary figure that
for lower middle income country per capita GDP as of 2011 should be 1876.68 USD? If this is the case neither
Pakistan is lower middle income country nor India even with 1500 USD per capita. They also not likely to be one by 2021 with 3335 USD per capita. So, it is not a big deal if Bangladesh also can not be if we go by your logic.
Even under present circumstances Bangladesh within a decade will cross per capita GDP of Pakistan. If we accept your argument even with that Bangladesh wont be middle income country by 2021 then what will be the condition of Pakistan???
Now lets see what is the definition of countries by certain income level. I am using World Bank's definition that you are saying have used here.
How we Classify Countries | Data
Income group: Economies are divided according to 2011 GNI per capita, calculated using the World Bank Atlas method. The groups are: low income, $1,025 or less;
lower middle income, $1,026 - $4,035; upper middle income, $4,036 - $12,475; and high income, $12,476 or more.
Now tell me in which angle it appeared to you Bangladesh wont cross the $1026 per capia income mark by 2021.
What is appearing to me you are using average income of all middle income countries and coming up with that value. But that is irrelevant here.