How many Muslims were left in northern India after partition? A communist southern threat could make Pakistan reunifying a possibility.
Few points that will ensure no merger/reunification IMHO:
a. Democratic (North India) as an entity will still be larger than Pakistan and Muslims a non-concentrated minority
b. Muslims in India (external to Kashmir at least) will feel Indian so I very much doubt they'd even entertain such a thought.
c. Indian identity itself would still be cemented by the 1950s so for majority of North Indians (Hindus, Sikhs) the focus will shift to REUNIFICATION OF INDIA under one capitalist republican banner as opposed to looking north (Communists will be focussing on the reverse - to 'reclaim' New Delhi).
d. International (Western) pressure will ensure Pakistan abides by all previous international borders, treaties and agreements it made at independence.
e. F**k knows what butterflies this will create in terms of alliances, blocs, external influence in both Indias. My assumption is South India will look to Moscow while North India will be unaligned or look to Washington (?) thus a merger with Pakistan isn't really needed anyway as you'll have 2 pro-Western governments side by side.