i am thinking what will happen when block halving reward will become negligible may be after 2050 or before that. even now itself miners had quit mining because profit dropped from 25 to 12.5 then think of 2050's situation.
That is a wrong statement. Before halving the exchange rates were around $230 per coin. So for every block, they were getting the equivalent of $5,750. And now, after the halving they are getting $740 per coin, for a total of $9,250. Actually their profits have increased by around 80%.
Historically that may be true, but as the reward continues to divide, I don't think that miners will continue to make the same amount. It would require the price of bitcoin to at least double every reward halving.