My view is that the numbers were not significantly higher. It just that these black holes were still in the strongly mass absorbing stage of their life, and therefore can be easily seen. After they absorb everything in their neigborhood, they settle down to a very long invisible life. The only mechanism we know of to diminish their size is Hawking radiation, which for relatively small and all larger black holes is much too weak to be detected. Only tiny black holes radiate much just before they disappear.
Black holes are forever.
That is the destiny of the universe after all the stars burn out. Chandra now holds the record for looking back in time, with the exception of COBE observing the universal background radiation, an observation that does not yield an age. From Chandra the age of the universe is now pushed back to 13-15 billion years. In another 10 billion years only black holes will be left.