The scenario you described, where all black holes merge into one, causing a new Big Bang and swallowing the entire universe, is not supported by current scientific understanding.
Firstly, black holes merging into one is a possibility in the future, but it would occur over an extremely long timescale. Black hole mergers have been observed and confirmed through the detection of gravitational waves, and it is believed that as the universe ages, more black hole mergers will occur. However, the timescales for such mergers are incredibly vast, spanning billions and trillions of years.
Secondly, the idea of a black hole swallowing the entire universe and leading to a new Big Bang is not in line with our current understanding of cosmology. The Big Bang is thought to be the initial event that marked the beginning of our universe, and it is not expected to be caused by black hole mergers. The Big Bang was a rapid expansion of space itself, not the result of a collapse or merger of matter.
Furthermore, the ultimate fate of the universe is still an active area of research and speculation. Depending on the amount of matter and dark energy in the universe, it could continue expanding indefinitely, reach a state of equilibrium, or experience a contraction known as the Big Crunch. The specific outcome will depend on various factors and is still being investigated.
In summary, while black hole mergers can occur over vast timescales, the notion of a single black hole swallowing the entire universe and leading to a new Big Bang is not supported by current scientific understanding. Our understanding of the universe's ultimate fate and the events that might lead to its end is an ongoing area of study and exploration.