Likely progression is simply that food gets expensive, and is grown indoors. Technology doesn’t need to fall, though when slaves are not needed, soylent green is a “utilitarian” use for them under rules based world order. Food capacity and population that can afford to buy it will match. Fewer people does mean fewer iphones, and more expensive at lower scale.

Global warming, even at 5C, is more about increased misery and oppression, rather than mass deaths over a decade. Wikipedia will survive. The AI tech giants chatbots will explain why you need to die or be miserable until you die.