Remember a joker called Raj Narain defeated Smt.Indira Gnadhi . At one point of time congress was thought to have been washed out of Andhra , in the high tide of Telugu desam, and in the very next elections it came back to power with thumping majority. At one time it was thought by political pundits that Congress is standing on its toes in the entire South, but regained a foot hold as much quickly.
You can not really say what happens when , it is all politics. How can you think that these kit-kat political parties who depend on single individual can hold on for any longer time?
Look DMK is Karunanidhi, Telugudesam Chandrababu Naidu, SP Mulayam, BSP Mayawati, Lokdal Ajit and so on are dependant on single inviduals with no history to speak and no idealogy to back them. For example when Lallu Yadav is gone his party is gone ! So it would be with SP and Mulayam.
In the political arena this vaccum has come in the absense of a charismatic crowd puller in the Congress party. BJP,and Communist parties difinitely survive as they have ideology,history and a base or reason to survive. Congress party has a luminous history,and also now it is in power. Sonia and Rahul can not be written away, and Congrees always stayed together in the event of crises at least so far. They found three leaders one Lalbahadur shastri after Nehru( a crises)and PV Narasimha rao after Rajiv Gandhi. And now Manmohan Singh. Congress and BJP are the major parties in next election.