Reasonably one can argue that the death of old ideas leads to new ones but there is a process akin to mourning when a cherished belief is killed off. Learning new ideas also includes learning how to discard old knowledge that is no longer relevant or just plain wrong; in fact, this sort of learning is a necessary skill set. Knowledge changes and evolves as human beings mature and gain life experience. The ideas of a five-year-old are not relevant to the ideas of an eighteen-year-old, nor should they be. Our decades as adults are also dynamic and ideas will continue to drop and add through the years. Saying goodbye to immature ideas is part of the human process of growth.
Ideas actually have parts to them. They begin with assumptions that are taken as true simply because they exist. From the assumptions come the arguments, the justifications of the idea. The last part is the conclusions, the idea itself that appears solid because the assumptions are true and the arguments are sound. When most people talk about ideas, they take the assumptions and arguments for granted and speak only of the conclusions. However, when the soundness of an idea is in question, it is the assumptions and the arguments that are reviewed.
The idea of God is subject to the same rules of assumptions and arguments. In the Jewish, Christian and Islamic worlds, the arguments for the belief in God have been disproved over and over again. Sometimes the weakness is the argument; however, the fatal flaw in all of the ideas about God is the assumptions. (See the work of Immanuel Kant.)
When the assumption is wrong, everything that follows from it is also wrong. A loss of a belief of this depth requires a fundamental reshaping of how a person approaches and operates in the world. The old responses are now empty and hollow because they no longer make sense. While the philosopher can hide in intellectualism and scientist can stand behind rationalism, most of us adhere to the principle of reasonableness, which includes the non-intellectual components of human thought such as emotions. For reasonable people, losing a cherished idea such as a God who is personal and personally involved, is also painful, something that may require mourning.
Ideas are not purely rational or utterly intellectual, contrary to the rigid strictures of the day. There is an interiority* to our search for ideas that resound. They have to also satisfy some need in our interior life or at least offer balm to the questions of why? and why not? of our deepest thoughts. That an idea would satisfy our cravings for answers about the meaning of life or the meaning of our existence is . . . reasonable.
Reasonable as a criterion is much more personal, more personally painful than purely logical arguments. They are also much more satisfying too.
*Philosophy term indicating inner dialogue and reflective thinking of human beings.