Why does the author regret the demise of the "old," more brutal gods?

Addressing the ancient Roman goddess Proserpine on the eve of Rome’s conversion to Christianity, the speaker laments the passing of the old gods. The speaker does not trust those who promise that the new gods, or the new religion, will be an improvement. Although C. A. Swinburne establishes the setting of the poem in the past, he is offering an allegory of his own time and place, Victorian England. The author apparently posits a comparison with the waning of Christianity in a new era when faith in science and rationalism is replacing that in a Christian god. While the author himself was critical of the conventional restrictions of the Anglican church, he also harbored doubts about the benefits of a secular worldview.
Of the old Roman gods, the speaker says, “they are cruel as love or life, and lovely as death,” and regrets that the people have so quickly “dethroned” them, rendering them “wiped out in a day!” If a belief system is valid, they imply, it would likely be more durable. It seems that other reasons may account for the religion’s demise, and the speaker is suspicious of the explanations that the advocates of the new have given, such as liberation “from your chains,” which brings mercy, pity, and compassion. The speaker finds these new gods “barren” and sees that they are taking away the joy of living life on earth. This can be interpreted as a lack of faith in the redemptive power of Christ and the hope of resurrection, with which Swinburne apparently found fault philosophically. In an extended metaphor, the speaker compares life to the ocean, noting the many ways that the waves buffet the rocks that hold fast: “Fate is a sea without shore, and the soul is a rock that abides . . . .”
The speaker engages in another long comparison, this time of Proserpine to the Virgin Mary, pointing out that her deified status came about as mother to a mortal who then proved to be divine. The speaker is more trusting of gods that are complete in their own divinity, as separate from mortal beings. They go back to the time when death brought finality, which seems to them the correct course of progression. “I know/I shall die as my fathers died, and sleep as they sleep; even so.”
https://www.poetryfoundation.org/poems/45292/hymn-to-proserpine-after-the-proclamation-in-rome-of-the-christian-faith